Wallace effect

Abstract: In considering the origin of reproductive isolation, two theories, which are not mutually exclusive, have been advanced. Muller (1939) considers that reproductive isolation between two subspecies is just a consequence of their genetic divergence following selection in geographic isolation. In this way, as the divergence proceeds, genes that prevent free interbreeding will be incorporated into the genotypes of the two subspecies. On the other hand, Dobzhansky (1940) holds that divergence does not necessarily imply reproductive isolation. In becoming more adapted to their particular environments, each subspecies may diverge far enough so that hybrids between them are less well adapted to the available habitats than either parental subspecies. Natural selection then will work to build up a gene system for further reproductive isolation. The formation of ill-adapted hybrids is prevented, avoiding in this way a wastage of reproductive potential and food resources. Grant (1966) proposed that the process of selection for reproductive isolation be designated as the Wallace effect. In animals, there are some interesting experiments to test the origin of reproductive isolation. Koopman (1950) was able to demonstrate a marked intensification of the reproductive isolation between Drosophila pseudaoobscura and D. persimilis as a result of selection. The experiment was conducted below 16 C where little isolation between these two species prevails. Using stocks marked with different recessive genes, all hybrid offspring could be readily detected since they carried both dominant genes. All hybrid progenies were then eliminated in every generation, and in a few cycles of selection the isolation increased substantially. Using a similar technique, Knight et al. (1956) attempted to develop sexual isolation between two stocks within a species. Experiments were conducted with D. melanogaster for 33 to 38 generations and a partial sexual isolation was obtained. Experimental studies on the origin of reproductive isolation in plants are almost completely lacking. The present paper deals with the results obtained after selecting for intraspecific reproductive isolation between two varieties of maize. Different recessive marker genes were used in the two populations. Hybrids, resulting from heterogamic fertilization, were readily recognized and discarded in every generation. Since Zea mays L. has, perhaps, more genetic variability, it seems that except for the generation interval, maize is better material for such a study than is Drosophila. In fact, within the single species Zea mays L. over 250 races have been described as occurring in different areas of the Americas (Wellhausen et al., 1952, 1957; Hatheway, 1957; Roberts et al., 1957; Brieger et al., 1958; Brown, 1960; Grobman et al., 1961; Ramirez et al., 1961; Timothy et al., 1961, 1963). Some of these races are quite ancient, about 4000 years old, and some are of recent origin, being the result of hybridization among more ancient ones. Many kinds of variation in 1 Supported in part by grants from the Program in Agricultural Sciences of The Rockefeller Foundation. 2 This paper is part of the homage the Brazilian Society of Genetics pays to Prof. Th. Dobzhansky on his 70th birthday in acknowledgment of his effective role in developing genetics in Brazil.

Abstract: All hermaphrodite plants of the Cirsium hybrids had viable pollen, generally lower than found in pure species. The pollen viability of a hybrid generally decreased with increasing genetic distance between the parents and when the parental species had lower pollen viability. The pollen viability was decreased in frequently hybridizing species where occasionally individuals of pure species morphology may show decreased pollen viability. In populations of gynodioecious species where females co-occurred, pollen viability (in hermaphrodites) was lower, indicating inbreeding depression. Hybrids between sympatric species exhibited higher post-pollination isolation (decrease of pollen viability), which suggests that the reproductive isolation had been increased by natural selection (effect similar to the Wallace effect). The strength of the postzygotic barrier (based on pollen viability) was generally stronger than that of the prezygotic barrier (based on distribution overlap) in hybridizing species pairs.

Abstract: Hybridisation is common in plants and can affect the genetic diversity and ecology of sympatric parental populations. Hybrids may resemble the parental species in their ecology, leading to competition and/or gene introgression; alternatively, they may diverge from the parental phenotypes, possibly leading to the colonisation of new ecological niches and to speciation. Here, we describe inflorescence morphology, ploidy levels, pollinator attractive scents, and pollinator guilds of natural hybrids of Arum italicum and A. maculatum (Araceae) from a site with sympatric parental populations in southern France to determine how these traits affect the hybrid pollination ecology. Hybrids were characterised by inflorescences with a size and a number of flowers more similar to A. italicum than to A. maculatum. In most cases, hybrid stamens were purple, as in A. maculatum, and spadix appendices yellow, as in A. italicum. Hybrid floral scent was closer to that of A. italicum, but shared some compounds with A. maculatum and comprised unique compounds. Also, the pollinator guild of the hybrids was similar to that of A. italicum. Nevertheless, the hybrids attracted a high proportion of individuals of the main pollinator of A. maculatum. We discuss the effects of hybridisation in sympatric parental zones in which hybrids exhibit low levels of reproductive success, the establishment of reproductive barriers between parental species, the role of the composition of floral attractive scents in the differential attraction of pollinators and in the competition between hybrids and their parental species, and the potential of hybridisation to give rise to new independent lineages.

Abstract: Seed set, seed germlnatlon and hybrld pollen fertilltles following controlled pollinations of 4828 flowers were investigated in the 12 species of kangaroo paws to assess taxonomlc and evolutionary relationships, document breeding systems and explore the potential and limitations in syntheesizing horticulturally desirable hybrlds in the group. All species except Anigozanthos flavidus set few seeds on selfing relative to numbers set on intrapopulational crossing, and thus were predominantly outbreeding. Interpopulational crossing barriers in A humilis were usually non-existent. In A. viridis they occurred between races in a geographical pattern consistent wtth the occurrence of the Wallace Effect (reproductive character displacement) In A. bicolor they were complex and generally uncorrelated wlth interpopulational distances or morphological (racial) divergence, while in A. manglesii they were associated consistently wlth racial differences. Interspecific crosses involving 79 species combinations and 215 population combinations invariably revealed crossing barriers more potent than those seen in intraspecific crosses. The biosystematic data support the taxonomic conclusions that Anigozanthos and Macropidra are genetically isolated genera, that recognition of just two sections (Anigozanthos and Hapianthesis) represents the best subgeneric classification of Anlgozanthos and that members of the A. bicolor-A. gabrielae, A viridis, A humilis-A kalbarriensis and A rufus-A pulcherrimus groups each constitute distinct species rather than intraspecific taxa The majority of Interspecific F1 hybrids were found to succumb to fungal attack, snail predation and/or inappropriate watering within 2 or 3 years of cultivation Hybrids of A. flavidus appeared to be the most vigorous, long-lived and floriferous, and probably are the most suitable for large-scale horticultural development