Showing papers in "Plant Species Biology in 1990"
TL;DR: Outcrossing rate variation in E. paniculata is controlled by a complex interplay of local demographic and genetic factors and influences operate within contrasting regional contexts as a result of the different evolutionary histories of populations in N. Brazil and Jamaica.
Abstract: A survey of 32 populations of the self-compatible, tristylous, aquatic, Eichhornia paniculata (Pontederiaceae), from N. E. Brazil and Jamaica, was undertaken to examine the role of demographic and reproductive factors on variation in outcrossing rate (t), and to investigate the association between t and population genetic structure. Multi-locus outcrossing rates, estimated using isozyme techniques, varied widely (t= 0.002–0.960) among populations and were uniformly distributed across the entire range of t. Sixty percent of variation in t was explained by style morph diversity (E) and the frequency of selfing variants within populations (M). Population size (N) and plant density (d) also accounted for a significant portion of variation in t, particularly in Jamaica, where variation in style morph diversity was low.
Outcrossing rates were significantly correlated with the proportion of loci that were polymorphic (P), the average number of alleles per polymorphic loci (Na), the average observed heterozygosity, (Ho), and the inbreeding coefficient (f). A strong regional effect was evident in the association of f with P and Na. This largely results from the confounding effect of genetic bottleneck(s) and high levels of self-fertilization associated with the colonization of Jamaica. Comparisons of f with the equilibrium inbreeding coefficient, (Feq), indicated that outcrossing populations had a significant deficiency of heterozygotes, while selfing populations had an excess. This demonstrates that levels of heterozygosity cannot be accounted for by the mating system alone. Outcrossing rate variation in E. paniculata is controlled by a complex interplay of local demographic and genetic factors. These influences operate within contrasting regional contexts as a result of the different evolutionary histories of populations in N. E. Brazil and Jamaica.
148 citations
TL;DR: It seems likely that diploid homosporous pteridophytes favor gametophytic crossing with some exceptions favoring gametopytic selfing as a derived condition, whereas polyploid Homospora pterids favor gametic selfing predominantly.
Abstract: Previous studies on the sequence of the gametangium formation in homosporous fern gametophytes (Masuyama, 1975a, b) suggested that diploid and polyploid taxa may favor gametophytic crossing and gametophytic selfing, respectively. In this view, intraspecific polyploids of three fern species (Phegopteris decursive-pinnata, Lepisorus thunbergianus and Pteris dispar) have been investigated for the amount of genetic load or the rate of gametophytic selfing. As expected, a marked contrast of mating systems is found between diploids and tetraploids; the former predominantly undergo gametophytic crossing and the latter predominantly undergo gametophytic selfing. It seems likely that diploid homosporous pteridophytes favor gametophytic crossing with some exceptions favoring gametophytic selfing as a derived condition, whereas polyploid homosporous pteridophytes favor gametophytic selfing predominantly.
84 citations
TL;DR: In contrast to evidence for inbreeding depression, there was very little evidence that resource allocation, sex lability, or habitat partitioning have played an important role in the evolution of dioecy.
Abstract: The evolution of dioecy was studied in Schiedea (Caryophyllaceae), a genus endemic to the Hawaiian Islands. Eight of the 22 species are diclinous, possessing gynodioecious, subdioecious, or dioecious breeding systems. A biogeographic analysis of the genus indicates that the ancestor of Schiedea colonized early in the history of the Hawaiian Islands. Subsequently, hermaphroditic species appear to have engaged in inter-island colonization more frequently than diclinous species. For this reason, single-island endemism and dicliny are more common on the older Hawaiian Islands. Strong inbreeding depression was detected in three species of Schiedea, indicating that genetic factors have played a role in the evolution of dicliny. Depending on the level of natural selfing, the expression of inbreeding depressioin may have favored the outcrossed progeny of rare females in populations, and eventually the evolution of dioecy. In contrast to evidence for inbreeding depression, there was very little evidence that resource allocation, sex lability, or habitat partitioning have played an important role in the evolution of dioecy. In subdioecious S. globosa hermaphrodites were largely male in function, and in gynodioecious S. salicaria females and hermaphrodites were equivalent in nearly all aspects of female function that could be measured. Variation in breeding systems in Schiedea and the closely related Alsinidendron may result from the past history of population bottlenecks that have resulted in varying levels of inbreeding depression.
44 citations
TL;DR: Results can be drawn that sexual and agamospermous relatives are usually ecologically differentiated; sexuals tend to require extreme habitats, while agamOSperms prefer disturbed places and part of this advantage is attributed to polyploidy.
Abstract: Recent progress in research on the evolution of agamospermy in Boehmeria (Urticaceae) and Eupatorium (Asteraceae) is reviewed Specific questions addressed are: (1) Is there any trend in distributions and habitat preferences of sexual and agamospermous races? (2) Is hybridity or a high level of heterozygosity a prerequisite for the evolution of agamospermous races? (3) Do agamospermous plants produce genetically variable offsprings through hybridization with sexual relatives? and (4) Do agamospermous plants have the theoretically predicted reproductive advantage over sexuals? From the results presented, the following conclusions can be drawn (1) Sexual and agamospermous relatives are usually ecologically differentiated; sexuals tend to require extreme habitats, while agamosperms prefer disturbed places (2) Hybridity or a high level of heterozygosity is not a prerequisite to the evolution of agamospermous races (3) Agamospermous races produce genetically variable offspring through hybridization with sexual relatives and subsequent segregation (4) Agamospermous plants have a reproductive advantage often greater than twofold; part of this advantage is attributed to polyploidy The question of why agamospermous races do not replace sexuals remains open Male recovery in agamosperms through hybridization and/or the disadvantage of polyploidy associated with agamospermy might work as mechanism(s) enabling sexuals to resist colonization of reproductively superior agamosperms under some extreme ecological conditions
42 citations
TL;DR: A mimicry hypothesis is proposed in which the non-rewarding female- stage flower mimics the rewarding male-stage flower to explain the short life-span of an individual flower, the long flowering period of anindividual tree, and the coexistence of flowers of various stages on a single tree.
Abstract: The floral biology of Magnolia hypoleuca, a tall tree species widely distributed in Japan, was studied in a deciduous broad-leaved forest in Hokkaido near the northern limit of distribution of the species. The flower is large, protogynous and nectarless. Its petals close after the female stage and again after the male stage. The mean duration of flowering of an individual flower is 3–4 days, while the flowering period of an individual tree lasts up to 40 days. The main pollinators of the flowers were beetles. Success of pollination is assumed to be affected by weather conditions at anthesis. Average fruit sets (ratios of fruits to flowers) were 13–25%. Hand-pollination of 44 flowers produced a significantly higher fruit set (28 fruits) than control flowers (33/133). These results suggest that the fruit set is limited by pollination. The frequency of infertile ovules was higher in controls than in hand-pollinated fruits, indicating that the seed set was also limited by pollination. We propose a mimicry hypothesis in which the non-rewarding female-stage flower mimics the rewarding male-stage flower to explain the short life-span of an individual flower, the long flowering period of an individual tree, and the coexistence of flowers of various stages on a single tree.
35 citations
TL;DR: The most noteworthy finding was that the size of chasmogamous seeds sharply decreased in response to a decrease in the light regime, while cleistogamous seed size remained constant along the gradient, reflecting different degrees of environmental stress, biotic interference, or both.
Abstract: The process and mechanisms of spatio-temporal changes in growth, population structure, as well as various yield and reproductive components of a population of an amphicarpic annual, Polygonum thunbergii (Polygonaceae), with two reproductive systems (aerial chasmogamous flowers and subterranean cleistogamous flowers) along an environmental gradient (light, moisture and soil nitrogen levels) were investigated in the field.
The results clearly demonstrate that growth and allocation patterns, population structure, and reproductive output of individuals changed sharply along the environmental gradient in response to seasonal and spatial changes in resource availability. Models predicted that light conditions bring about one-sided competition, whereas nutrient conditions in the soil engender two-sided competition. As expected, the degree of one-sided competition was prominent in the case of a planophile, Polygonum thunbergii. Allocation patterns, seed outputs, individual seed size as well as relative energy costs of chasmogamous and cleistogamous seed as affected by light and nutrient levels were also critically analyzed. The most noteworthy finding was that the size of chasmogamous seeds sharply decreased in response to a decrease in the light regime, while cleistogamous seed size remained constant along the gradient. However, relative cost of both chasmogamous and cleistogamous seeds sharply increased with decrease in the light level, reflecting different degrees of environmental stress, biotic interference, or both.
35 citations
TL;DR: The investigations of the significance of agamospermous triploid pollen donors in the sexual relationships between diploids and triploids in Taraxacum are reviewed and some examples suggest that agamOSpermous polyploids can increase their genetic diversity through obtaining genes from coexisting diPLoids.
Abstract: We review in this article the investigations of the significance of agamospermous triploid pollen donors in the sexual relationships between diploids and triploids in Taraxacum. Crossing experiments between diploid sexual mother plants and agamospermous polyploid pollen donors and recent isozyme analyses of the progeny have exhibited the following results: 1) Pollen from Agamospermous polyploid pollen donors have the potential to give rise to the polyploid agamospermous offspring when fertilizing diploid sexual plants. Ploidy level of the progeny is usually the same or higher, but occasionally lower, compared to the pollen donor. 2) Diploid progeny also occur from diploid (♀)-polyploid (♂) crosses, however, these diploids were in our results not hybrids but the results of self-fertilization of the diploids. The self-fertilization is regarded as a cosequence of the breakdown of the self-incompatibility barrier through the sterile triploid's pollen. This breakdown is in all probability a common phenomenon in diploid (♀)-polyploid (♂) crosses. Some examples suggest that agamospermous polyploids can increase their genetic diversity through obtaining genes from coexisting diploids. The evolutionary implications of this phenomenon and the reduction mechanism of chromosome number through agamospermous pollengenesis are discussed.
33 citations
TL;DR: Models of the joint evolution of selfing and inbreeding depression predict either strong outcrossing or predominant selfing, although wind-pollinated plants fit the prediction, some animal- pollinated species have intermediate selfing rates, refuting the theory.
Abstract: First, evolutionary theories of selfing of terrestrial plants are reviewed briefly The evolution of the selfing rate is controlled mainly by (1) the benefit of enhanced genetic relatedness to seeds and (2) the cost of lowered fitness of selfed offspring (inbreeding depression), being modified by (3) fertility assurance under pollen limitation, (4) reduced performance as pollen donor, (5) reduced expenditure to male function, and (6) lowered genetic recombination Models of the joint evolution of selfing and inbreeding depression predict either strong outcrossing or predominant selfing Although wind-pollinated plants fit the prediction, some animal-pollinated species have intermediate selfing rates, refuting the theory
Second, three resource allocation models are analyzed, in which an individual plant optimally allocates limited resources to outcrossed seeds, selfed seeds, and to energy reserves for the next year The first model explains how the number of outcrossed and selfed offspring change with plant size when they differ in dispersal distance The second model predicts that, in a disturbed habitat, the plant is likely to be annual and to produce both selfed and outcrossed seeds; in contrast, in a stable habitat, the plant tends to be perennial and to abort selfed seeds selectively Hand pollination may increase seed production for perennials but not for annuals The third model explains the observed difference between animal and wind pollinated plants in the out-crossing rate pattern by the difference in the way pollen acquisition increases with investment
30 citations
TL;DR: The pollination ecology of Pedicularis punctata was studied in the Pir Panjal Range of the Kashmir Himalaya in the summer of 1989 and its nectarless, rostrate, long-tubed flower was found to be pollinated exclusively by Bombus foragers vibrating pollen while the stigma contacted pollen in the pollinator's cervical crevice.
Abstract: The pollination ecology of Pedicularis punctata was studied in the Pir Panjal Range of the Kashmir Himalaya in the summer of 1989. Its nectarless, rostrate, long-tubed flower was found to be pollinated exclusively by Bombus foragers vibrating pollen while the stigma contacted pollen in the pollinator's cervical crevice. Workers of Bombus tunicatus and B. flavothoracicus comprised 95% of its pollinators. Pollen-foraging fidelity of its pollinators was greatest where diversity of Bombus-pollinated plant species in three plant communities was least. Foragers on other plants carried virtually no Pedicularis pollen. P. punctata is a mid-season blooming species similar in its pollination syndrome to comparable species in other geographic regions. The enigmatic function of its long, nectarless corolla tube, even more exaggerated in other Asiatic species, requires further investigation.
25 citations
TL;DR: A hypothesis is proposed that the mean and variance of pollinator availability together with inbreeding depression could explain the observed patterns of mating system as the results of evolution.
Abstract: Island populations of Campanula microdonta and mainland Honshu populations of C. punctata have several different mating systems: self-incompatible (SI) and therefore obligately outcrossing in mainland Honshu and Oshima Island; self-compatible (SC) and largely outcrossing in the northern islands of Toshima and Niijima; and SC and predominantly inbreeding southern ones of Miyake and Hachijo. Several features possibly associated with the mating systems are described. A hypothesis is proposed that the mean and variance of pollinator availability together with inbreeding depression could explain the observed patterns of mating system as the results of evolution.
24 citations
TL;DR: These seed dormancy/ germination characteristics of L. stonensis do not differ from those reported for some geographically widespread, weedy species of winter annuals and thus do not help account for the narrow endemism of this species.
Abstract: Lesquerella stonensis (Brassicaceae) is an obligate winter annual endemic to a small portion of Rutherford County in the Central Basin of Tennessee, where it grows in disturbed habitats. This species forms a persistent seed bank, and seeds remain viable in the soil for at least 6 years. Seeds are dormant at maturity in May and are dispersed as soon as they ripen. Some of the seeds produced in the current year, as well as some of those in the persistent seed bank, afterripen during late spring and summer; others do not afterripen and thus remain dormant. Seeds require actual or simulated spring/summer temperatures to come out of dormancy. Germination occurs in September and October. Fully afterripened seeds germinate over a wide range of thermoperiods (15/6–35/20°C) and to a much higher percentage in light (14 h photoperiod) than in darkness. The optimum daily thermoperiod for germination was 30/15°C. Nondormant seeds that do not germinate in autumn are induced back into dormancy (secondary dormancy) by low temperatures (e.g., 5°C) during winter, and those that are dormant do not afterripen; thus seeds cannot germinate in spring. These seed dormancy/ germination characteristics of L. stonensis do not differ from those reported for some geographically widespread, weedy species of winter annuals and thus do not help account for the narrow endemism of this species.
TL;DR: Data suggest that non-random mating among compatible donors is a relatively common process in wild radish through mechanisms controlled by the pollen tubes, the maternal plants or the embryos and whether sexual selection actually occurs and how important it may be is still nuclear.
Abstract: When mating is non-random among several, compatible donors, the fitness of pollen donors, maternal plants, and offspring may be affected. Although this process may be important, it is much less studied than other forms of non-random mating such as incompatibility and avoidance of inbreeding. Therefore, the amount and consequences of non-random mating were investigated in greenhouse studies with wild radish, Raphanus sativus. Six compatible donors differed in the number, position, and weight of seeds sired, so mating was non-random at the level of mate identity. Mate number also affected mating patterns; fruits with more fathers were allocated more resources. This keeps mate number per fruit high. In contrast, other processes appear to keep mate number below the maximum so that mate number per fruit is regulated at an intermediate level. Mate identity had clear consequences as offspring with different fathers were of different sizes after 11 weeks. The effects of mate number on offspring success were less clear. These and other data suggest that non-random mating among compatible donors is a relatively common process in wild radish. It may occur through mechanisms controlled by the pollen tubes, the maternal plants or the embryos. While this non-random mating is the raw maternal for sexual selection in plants, whether sexual selection actually occurs and how important it may be is still nuclear.
TL;DR: Overall results on the genetic structure of the D. tokoro population indicated a high gene flow among its subpopulations, and this may be the consequence of its life form as a climber and its habitat in a disturbed environment.
Abstract: Dioscorea tokoro Makino is a herbaceous climber species widespread in East Asia. Genetic structure of a natural population of D. tokoro was examined employing starch gel electrophoresis of allozymes. Genotypes of seven loci were studied for 1,128 individuals. Twenty-six populations located mainly in the Kinki district of Japan were subgrouped into four large clusters by the geographical distribution of alleles. The D. tokoro population was revealed to contain greater total genetic diversity (HT=0.282) and higher intrapopulational genetic diversity (HS=0.258) than other outcrossing species for which data are available. On the other hand, interpopulational differentiation (GST=0.096) was smaller than in other outcrossers. For the heterozygosity deficiency observed (FIT=0.125), population subdivision (FST=0.096) and inbreeding within the population (FIS=0.067) were revealed to contribute to the same extent. From these F-statistics, the migration rate among subpopulations and the rate of between-relative matings were estimated. Overall results on the genetic structure of the D. tokoro population indicated a high gene flow among its subpopulations, and this may be the consequence of its life form as a climber and its habitat in a disturbed environment. During the study, the geographical cline of Pgi allele frequencies was observed. This finding was supposed to be the result of the selection imposed on Pgi by the temperature differences between localities.
TL;DR: Dichogamy and sex allocation in several populations of Campanula microdonta and C. punctata were investigated with regard to their mating systems, finding that small flowers allocate proportionately more resources to the androecium than to the gynoecium within each population.
Abstract: Dichogamy and sex allocation in several populations of Campanula microdonta and C. punctata were investigated with regard to their mating systems. Duration of the staminate phase differed among the populations: staminate phase was longer in self-compatible (SC) and largely outcrossing populations than in self-incompatible (SI) and outcrossing populations or in SC and largely inbreeding populations. Duration of the pistillate phase among the populations was less variable than duration of the staminate phase. Male reproductive effort decreased with increase of the estimated selfing rates. Male allocation (weight ratio of androecium to gynoecium or to total flower) may be used as an indicator of the breeding system. Within each population, small flowers allocate proportionately more resources to the androecium than to the gynoecium. Among populations, SC outcrossing populations tend to produce large ovaries, and SC inbreeding populations tend to produce small ovaries.
TL;DR: The flower of the perennial shrubby herb, Hyptis suaveolens (L.) Poit is representative of an explosive pollination mechanism that requires explosion of carinal lobe for pollination.
Abstract: The flower of the perennial shrubby herb, Hyptis suaveolens (L.) Poit. is representative of an explosive pollination mechanism. The carinal structure formed by the middle lobe of corolla lower lip conceals the stamens and stigma and does not open following the natural anthesis. Upon anthesis, the carinal lobe is under tension which is later released by wind or bees. The violent explosion of the carinal lobe caused by wind results in self-pollination; by bees, it results in self- and cross-pollination. Foraging activity of some bees which collect pollen from the anthers and stigma on H. suaveolens may reduce the success of pollination.
The mating system involves both autogamy and allogamy. Both require explosion of carinal lobe for pollination. The flexibility of the mating system is considered a safeguard ensuring its ruderal nature for weediness.
TL;DR: It is argued that the evolution of large floral displays in milkweeds is the result of selection to increase overall reproductive success rather than male reproductive success alone.
Abstract: For hermaphroditic plant species whose fruit production is limited by maternal resources, the “pollen donation hypothesis” views large floral displays as an adaptation to enhance the probability of fathering seeds on other plants. This hypothesis has frequently been used to describe the evolution of large floral displays in milkweeds (Asclepias). Most tests of the pollen donation hypothesis, however, have used indirect measures, such as flower production or pollen removal, to estimate male reproductive success. To test the pollen donation hypothesis directly, we performed a paternity analysis and determined the number of seeds sired by individual genotypes in a natural population of poke milkweed, A. exaltata, in southwestern Virginia. Seeds sired (male success) and seeds produced (female success) were significantly correlated with flower number per plant (for male success: r= 0.32, P 0.05), indicating that plants with large floral displays did not reproduce primarily as males. Percent fruit-set and seed number per fruit were higher in 1986, when levels of pollinarium removal also were higher. Furthermore, several umbels that experienced high pollinator activity selectively matured fruits that contained many seeds. We argue that the evolution of large floral displays in milkweeds is the result of selection to increase overall reproductive success rather than male reproductive success alone.
TL;DR: Three Rubus species proved to possess similar perennation as well as reproductive strategies, although R. palmatus var.
Abstract: The reproductive characteristics of three Rubus species (R. palmatus Thunberg var. coptophyllus (A. Gray) O. Kuntze, R. microphyllus L.f., and R. crataegifolius Bunge) were critically investigated using materials collected from the forest floor of an 85-year-old Hinoki (Chamaecyparis obtusa) plantation in central Honshu, Japan. The underground branching patterns of these species were also investigated at the nursery. R. microphyllus produced the largest number of fruits in the same cane size among the three species (164 fruits per cane of 300 cm3 in D02H). However, the largest number of seeds produced per cane was in R. crataegifolius, where the maximum seed number attained 74,500 per cane, which was twice as many as those of the other two species. There was no apparent difference in dry matter allocation to fruits in these species. The three species produced numerous suckers by underground branching, forming large clones. In the open site at the nursery, the number of suckers produced by underground branching increased exponentially. On the other hand, in natural populations occurring in closed sites, the number of suckers produced by underground branching in the three species was by contrast much smaller. Three Rubus species proved to possess similar perennation as well as reproductive strategies, although R. palmatus var. coptophyllus shows much lower seed production and slower sucker production than the other two species.
TL;DR: Breeding experiments for four Japanese species suggest that, although they substantially possess the capability of both inbreeding and outbreeding, the high fecundity levels are maintained by predominant inbreeding system.
Abstract: The differentiation patterns of reproductive systems, including breeding and pollination systems, and perennation strategies of the genus Trillium were critically studied. The pedicellate- and sessile-flowered species with different floral morphologies and ecological distributions exhibited distinct modes of reproductive systems in relation to their habitat conditions. The pedicellate-flowered species occurring in stable mesic temperate deciduous forests reproduce exclusively by seeds, while sessile-flowered species growing in the alluvial flood plain habitats depend to a large extent on vegetative reproduction to compensate for the insufficient sexual reproduction under ecologically unstable conditions, e.g., frequent disturbance due to flooding. These sessile-flowered species are characterized by low ovule numbers, subsequent low seed outputs and low fecundity levels. In contrast to the sessile-flowered species, the pedicellate-erect-flowered species showed very high seed setting rates of over 50%. Breeding experiments for four Japanese species suggest that, although they substantially possess the capability of both inbreeding and outbreeding, the high fecundity levels are maintained by predominant inbreeding system. Furthermore, resource levels in the stock organs of these species obviously determine fecundity levels. As a consequence the amount of reserved food in the rhizomes transferred to the next season is determined, which gurantees a continuous reproductive activity of a given individual plant.
TL;DR: The evolutionary stability of the sex-changing habit of Arisaema serratum was examined and it was shown that in the size classes where sex change actually occurs, the reproductive values of males and females are basically the same.
Abstract: The evolutionary stability of the sex-changing habit of Arisaema serratum was examined. To evaluate reproductive success through male functioin properly, the mathematical formulation of Kakehashi and Harada (1987) was employed. Individuals were classified by both their size and sexual state, and the genetic contribution of individuals was estimated by calculating their reproductive value. It was shown that in the size classes where sex change actually occurs, the reproductive values of males and females are basically the same. This means that the genetic contribution of individuals in these size classes is the same whether they are male or female, and no selection pressure is working to change the male ratio in these classes. Thus, it is concluded that the size at which sex change occurs is evolutionarily stable. The adaptive significance of the size of first reproduction was also discussed.
TL;DR: The sex allocation theory based on size-specific demography is applied to sex change phenomena exhibited by Arisaema serratum, and reproductive values generally increased in accordance with size, but in a different manner for different sexes.
Abstract: The sex allocation theory based on size-specific demography is applied to sex change phenomena exhibited by Arisaema serratum, which changes sex expression according to its size. In the evolutionary analysis where adopted strategy correlates with resource level, identifying equal resource level is very important. But directly measured size does not seem to represent the plant's real resource level perfectly. In the analysis presented here, we introduce a new variable that represents the resource level. According to the sex allocation theory, we calculated the transition matrix and other demographic parameters, the stable sex-and-size distribution, and reproductive values. The obtained results were: (1) reproductive values generally increased in accordance with size, but in a different manner for different sexes; (2) in the size class where different sex expressions were adopted, reproductive values for the equal resources were the same; and (3) plant individuals seemed to adopt evolutionarily stable strategies. The adaptive nature of the plant is further discussed.
TL;DR: The shift from outbreeding to inbreeding appears to be an important key step in the occurrence of poliploids by hybridization between the different species.
Abstract: Species of Trillium have a disjunct distribution occurring in both North America and eastern Asia. In North America all 36 species are diploid. The 11 species of eastern Asia, however, include only a single diploid with all the other species being polyploids. Why do different patterns of speciation develop in North America and in eastern Asia? The breeding systems of populations in the North American T. erectum, T. grandiflorum and T. ovatum, and in Asian T. kamtschaticum were investigated by estimating the inbreeding coefficient from cold-induced banding patterns which reveal homozygotes and heterozygotes. From the analyses of the inbreeding coefficients, T. erectum, T. grandiflorum and the Pacific coastal species, T. ovatum are predominantly inbreeding species. T. ovatum populations from the Rocky Mountain region are outbreeders. However the Japanese species, T. kamtschaticum has a mixture of outbreeding and inbreeding among populations. The development of polyploid systems in Asia is possibly the result of the diversity of the breeding systems among the populations. The shift from outbreeding to inbreeding appears to be an important key step in the occurrence of poliploids by hybridization between the different species.
TL;DR: Two agamosporous fern species, Pteris cretica and Dryopteris yakusilvicola as well as some sexual species related to them were cytologically and enzyme electrophoretically analysed in order to elucidate their genetic variation and its origin.
Abstract: Two agamosporous fern species, Pteris cretica and Dryopteris yakusilvicola as well as some sexual species related to them were cytologically and enzyme electrophoretically analysed in order to elucidate their genetic variation and its origin. Pteris cretica in Japan has been reported to contain diploid and triploid agamosporous races. Our allozyme data strongly suggested that the agamosporous triploid originated through recurrent hybridization between the diploid agamosporous race of P. cretica and the related sexual species, P. kidoi. Recurrent hybridization with related sexual species can be considered to contribute to increase the genetic variation of this agamosporous species. On the other hand, triploid agamosporous species, D. yakusilvicola was suggested to be of hybrid origin between diploid sexual D. sabaei and the tetraploid sexual race of D. sparsa based on our allozyme data. Moreover, 56 individuals of D. yakusilvicola from various localities which were electrophoretically analyzed showed the same banding pattern. This genotypic uniformity suggests that this species was recently derived through a single hybridization event.
TL;DR: Reticulate evolution in homosporous ferns may be enhanced by high levels of intergametophytic crossing, and several mechanisms, including inbreeding depression, antheridiogen, and ontogenetic sequences that result in effectively unisexual gametophytes, promote outcrossing.
Abstract: Mating systems of 18 species of homosporous ferns follow a bimodal distribution, similar to that observed for seed plants (Schemske and Lande, 1985). Most species are highly outcrossing, a few are inbreeding, and two species examined to date have mixed mating systems. Equisetum arvense and several species of lycopods are also highly outcrossing. Several mechanisms, including inbreeding depression, antheridiogen, and ontogenetic sequences that result in effectively unisexual gametophytes, promote outcrossing in homosporous ferns and perhaps other homosporous pteridophytes as well. In some species of homosporous ferns, selection has favored the evolution of inbreeding as an adaptation for colonization. High levels of intra- and interpopulational gene flow via spore dispersal, coupled with high levels of intergametophytic crossing, generally lead to genetically homogeneous populations and species of homosporous ferns. However, rock-dwelling ferns and ferns from xeric habitats may exhibit significant population genetic structure due to physically patchy habitats. Reticulate evolution in homosporous ferns may be enhanced by high levels of intergametophytic crossing.
TL;DR: Isozyme study showed that both mating system and reproductive system jointly influence genetic structure of populations such as the level of intrapopulational variability and the extent of interpopulational differentiation.
Abstract: Variation in mating system in the common wild rice Oryza rufipogon was studied in relation to life-history traits to examine its effect on the genetic structure of natural populations. Results of general survey in Asian populations and of a detailed population study carried out in Thailand are presented. Estimated outcrossing rates of Asian wild rices ranged from 5 to 60%, though cultivated rices are predominantly selfing. Interpopulational comparison showed mating system is associated with life-history traits, resulting in the differentiation into two ecotypes; predominantly selfing annuals having high reproductive effort and mixed-mating perennials having low reproductive effort. Resource allocation to pollen production vs. seed production (pollination effort) proved to be correlated with outcrossing rate. Isozyme study showed that both mating system and reproductive system jointly influence genetic structure of populations such as the level of intrapopulational variability and the extent of interpopulational differentiation. Evolution of selfing in this plant group might have been promoted by selection for seed propagation which was brought by environmental change and/or habitat disturbance.
TL;DR: The breeding structure of these tropical species appears to be a mixture of near-neighbor and long-distance pollen movement, which has important implications for the genetic structure of populations as well as for the design of conservation areas.
Abstract: Several approaches were used to study the breeding structure of tropical tree populations located on Barro Colorado Island, Republic of Panama. Allozyme analyses of 16 woody species indicated that the distribution of genetic diversity among collection sites was closely associated with the species’breeding system and seed dispersal mechanism. Low levels of diversity observed among collection sites separated by 1–2 km indicate that intersite gene flow is high. Estimates of the proportion of outcrossing obtained for seven species indicated that the majority (6) were highly outcrossed. A significant amount of tree to tree heterogeneity in pollen allele frequencies also existed for each outcrossing species. Degree of heterogencity in pollen allele frequencies received by maternal trees was negatively associated with flowering tree density. In the mixed mating species, Cavanillesia plantanifolia, the proportion of selfing was closely related to flowering tree density. A significant proportion of the pollen received by individuals of these eight species came from relative few individuals. Pollen movement within populations was estimated for two canopy species. Long distance pollen movement (>750m) was extensive (>20%) in both species. The breeding structure of these tropical species appears to be a mixture of near-neighbor (30%–50%) and long-distance pollen movement (10%–25%). If this pattern is typical of tropical tree species, it has important implications for the genetic structure of populations as well as for the design of conservation areas.