Iris hexagona

Abstract: Phytohormones play critical roles in regulating plant responses to stress. We investigated the effects of salinity on abscisic acid (ABA), indole-3-acetic acid (IAA), salicylic acid (SA), and jasmonic acid (JA) in leaves, stalks, fruits, and seeds of Iris hexagona, a native wetland species. Using gas chromatography-mass spectroscopy with selected ion monitoring, our experiments demonstrated significant and different short- and long-term changes in iris phytohormones. ABA and JA generally increased and IAA and SA declined in response to salinity. We conclude that these phytohormones may have separate and interactive effects on how plants respond and adapt to stress in natural environments.

Abstract: Genetic variation present in 599 individual Iris samples was examined at 50 presumptive loci (isozymes). The samples included allopatric populations of I. fulva, I. hexagona, I. brevicaulis and the putative hybrid species, I. nelsonii. In addition, we examined two populations from areas of overlap involving I. fulva with I. hexagona, and I. fulva with I. brevicaulis and I. hexagona. Allozyme data for both of these hybrid populations suggest the presence of genes from I. fulva, I. brevicaulis and I. hexagona. Bidirectional introgressive hybridization is indicated between I. fulva and I. hexagona. Individuals from a parapatric association between these two species possess various combinations of I. fulva and I. hexagona diagnostic markers. The pattern of genetic variation in this sample indicates the presence of advanced hybrid generations and localized introgression; this finding is in accord with a previous molecular genetic analysis (Arnold et al., 1990). Furthermore, currently allopatric populations of I. fulva and I. hexagona were found to have low frequencies of marker alleles diagnostic for the alternate species. These populations also contain a low frequency of alternate, diagnostic ribosomal DNA repeat length variants (Arnold et al., 1990). The allozyme (and molecular) data suggest that these allopatric samples represent previously introgressed populations. Findings from the allozyme survey indicate that I. nelsonii contains an array of markers characteristic for I. fulva, I. hexagona and I. brevicaulis. Furthermore, some of these markers are found in a present-day hybrid population involving I. fulva, I. hexagona and I. brevicaulis. These findings suggest that I. nelsonii is of hybrid origin, deriving from hybridization between I. fulva, I. hexagona and I. brevicaulis. The genetic analysis along with previous chromosomal, distributional and demographic studies of the Louisiana Iris species suggests that a number of mechanisms may have been involved in the stabilization of this hybrid derivative.

Abstract: Genetic variation associated with the natural hybridization of Iris fulva and I. hexagona was investigated to test for the occurrence of introgression. These species have been viewed as a classic example of the process of introgressive hybridization (Anderson, 1949). However, more recent studies have concluded that there has not been an exchange of genetic material between I. fulva and I. hexagona (Randolph et al., 1967). Our analysis has involved the examination of both allopatric and parapatric populations of I. fulva and I. hexagona with reference to diagnostic ribosomal DNA markers. The pattern of variation in the parapatric population indicates the presence of the repeated backcrossing necessary to the process of introgressive hybridization. Indeed, in the region of parapatry, we suggest that localized introgression of ribosomal sequences has occurred into both I. fulva and I. hexagona. Significantly, we have also detected the presence of the diagnostic ribosomal markers from each species in allopatric populations of the alternate species. Our findings suggest that not only is introgressive hybridization presently occurring in parapatry between I. fulva and I. hexagona, but that past hybridization between these species has resulted in introgression into areas of allopatry.

Abstract: To elucidate the importance of hybridization in evolution, it is necessary to understand the processes that affect hybridization frequency in nature. Here we focus on postpollination, prefertilization isolating mechanisms using two hybridizing species of Louisiana iris as a study system. We compared the effects of differential pollen-tube growth on the frequency of F1 hybrid formation in experimental crosses between Iris fulva and Iris hexagona. Analyses of seed production in fruits from pure conspecific and heterospecific pollinations revealed that more seeds were produced in the top half than the bottom half of fruits for all four crosses. Heterospecific pollen was applied to flowers of each species at zero to 24 h prior to conspecific pollen, thereby giving a head start to the foreign pollen. Using diagnostic isozyme markers, the frequency of hybrid progeny was examined at the level of the whole fruit and separately for the top and bottom halves of fruits. In both species, the proportion of hybrid seeds per fruit increased significantly with increasing head starts, suggesting that differences in pollen-tube growth rates affect the frequency of hybridization. In I. fulva fruits, the increase in hybrid seeds occurred in both halves of the fruits, but in I. hexagona an increase was only detected in the top half of fruits. These findings are consistent with a model that assumes attrition of pollen tubes due to the greater length of I. hexagona styles. While pollen-tube growth rate appears to be the most important factor affecting hybridization frequency in I. fulva, both pollen-tube growth rate and pollen-tube attrition appear to be important in I. hexagona.