ASSORTATIVE MATING AND NATURAL SELECTION IN AN IRIS HYBRID ZONE

The phenology of different genotypes and the distribution of genetic variation among flowering plants and their progeny were examined to assess the levels of assortative mating and selection in a hybrid population of Iris. This study and a previous survey of RAPD nuclear markers and chloroplast markers indicate that the population consists of parental genotypes and recombinant hybrid genotypes that are similar to the parental species (I. fulva and I. brevicaulis), although lacking intermediate genotypes. Early in the season only I. fulva genotypes produced flowers, but as flowering in these plants decreased, the hybrid genotypes and I. brevicaulis genotypes began flowering, resulting in a 24-d period of coincidental flowering. The genotypic distribution of seeds produced during the period of flowering overlap contained a high frequency of intermediate genotypes that were not present in the adult generation. The degree of effective assortative mating was examined by comparing the observed progeny genotypic distributions with expected distributions from a mixed-mating model. The model included selfing and random outcrossing to the nearest plants that had pollen-bearing flowers on the day the recipient flower was receptive. The observed genotypic distribution of progeny from plants with I. brevicaulis chloroplast DNA (cpDNA) was not significantly different from the expected distribution. For I. fulva genotypes, however, there were higher than expected frequencies in the extreme genotypic classes, although intermediate genotypes were absent, indicating that these plants were preferentially mating with similar genotypes. Compared with the extreme genotypes, a larger proportion of the intermediate seed progeny produced were aborted, indicating that intermediate genotypes have lower viability. On the basis of the observed progeny genotypes and genetic disequilibria estimates for the adults and the progeny, there appears to be a pattern of effective asymmetrical mating in this population. This asymmetry is most likely due to pollen-style interactions that reduce the fertilization ability of genetically dissimilar pollen, or preferential abortion of genetically intermediate zygotes by I. fulva-like genotypes. The lack of any apparent discrimination by I. brevicaulis-like genotypes creates a directional exchange of nuclear genetic elements that will have implications for introgression and the evolution of hybrid genotypes.     

GENETIC INTERACTIONS AND NATURAL SELECTION IN LOUISIANA IRIS HYBRIDS

Hybridization between divergent lineages has long been assumed to give rise to unfavorable interactions between the parental genomes. These deleterious genetic interactions are further assumed to result in the production of hybrid offspring with decreased levels of viability and/or fertility. To test this assumption, we investigated the role of both nuclear and cytonuclear epistatic interactions in determining the frequencies of F₂ genotypes produced in crosses between two species of Louisiana iris, Iris fulva and I. brevicaulis. Overall, these crosses revealed a significant deficit of intermediate hybrid genotypes accompanied by an excess of parental-like genotypes, suggesting that genetic interactions may promote postmating reproductive isolation between these species. However, analyses of single and multilocus segregation patterns revealed a variety of negative and positive interactions between the genomes of the parental taxa at the nuclear and cytonuclear levels. Taken together, these results indicate that the traditional view that interactions between divergent genomes are always deleterious is an oversimplification. Rather, it seems likely that crosses between divergent lineages can lead to the production of both fit and unfit hybrid genotypes.     

HYBRID FITNESS IN THE LOUISIANA IRISES: ANALYSIS OF PARENTAL AND F1 PERFORMANCE

The assumption of hybrid inferiority is central to the two models most widely applied to the prediction of hybrid zone evolution. Both the tension zone and mosaic models assume that natural selection acts against hybrids regardless of the environment in which they occur. To test this assumption, we investigated components of fitness in Iris fulva, I. hexagona and their reciprocal F₁ hybrids under greenhouse conditions. The four cross types were compared on the basis of seed germination, vegetative and clonal growth, and sexual reproduction. In all cases, the hybrids performed as well as, or significantly better than, both of their parents. These results suggest that F₁ hybrids between I. fulva and I. hexagona are at least as fit as their parents. The results of this study are therefore inconsistent with the assumptions of both the tension zone and mosaic models of hybrid zone evolution.     

ECOLOGICAL AND GENETIC ASSOCIATIONS IN AN IRIS HYBRID ZONE

Chloroplast DNA (cpDNA) markers and 12 nuclear (random amplified polymorphic DNA, or RAPD) markers were used to examine the distribution of genetic variation among individuals and the genetic and ecological associations in a hybrid iris population. Plants in the population occurred at various distances from the edge of a bayou in a relatively undisturbed mixed hardwood forest and in an adjacent pasture dominated by herbaceous perennials with interspersed oak and cypress trees. The majority of plants sampled possessed combinations of markers from the different Iris species. Genetic markers diagnostic for Iris fulva and I. brevicaulis occurred at high frequencies, whereas markers diagnostic for I. hexagona were infrequent. For the majority of the nuclear markers, significant levels of cytonuclear disequilibria existed because of intraspecific associations among the markers in both the pasture and the forest. The distribution of nuclear markers among individuals was bimodal; intermediate genotypes were absent and the majority of RAPD markers were associated with their intraspecific cpDNA haplotypes. Strong intraspecific associations existed among RAPD markers in the forest, but associations tended to be weaker in the pasture area. Ecological correlations were detected for all but one of the I. fulva and I. brevicaulis RAPD markers. The ecological associations of hybrids similar to I. brevicaulis resembled associations of I. brevicaulis parental genotypes, suggesting that these hybrid genotypes may be relatively fit in the same habitats. The hybrids similar to I. fulva, however, were distributed in habitats that were unique relative to the parental species. The patterns of genetic and environmental associations along with other available data suggest that (1) only advanced generation hybrids were present in the population; (2) formation of F₁ hybrids among Louisiana irises is rare, leading to sporadic formation of hybrid populations; and (3) selection and assortative mating have contributed to the formation of hybrid genotypes that tend to be similar to parental genotypes. The patterns of ecological and genetic associations detected in this population suggest that assortative mating and environmental and viability selection are important in the structuring and maintenance of this hybrid zone.     

THE EFFECT OF HABITAT ON PARENTAL AND HYBRID FITNESS: TRANSPLANT EXPERIMENTS WITH LOUISIANA IRISES

We performed transplant experiments with Louisiana irises to test the assumptions of three models of hybrid zone structure: the bounded hybrid superiority model, the mosaic model, and the tension zone model. Rhizomes of Iris fulva, I. hexagona, and F₁ and F₂ hybrids were planted at four sites in southeastern Louisiana in 1994. Wild irises grew at all four sites, but differed in genotypic composition among sites. The sites were characterized by (1) pure I. fulva plants; (2) I. fulva-like hybrids; (3) I. hexagona-like hybrids; and (4) pure I. hexagona plants. The sites differed significantly in light availability, soil moisture and chemical composition, and vegetation. Survival of transplants was high in all sites and did not differ significantly among plant classes. Iris hexagona produced significantly more leaf material than I. fulva at the I. hexagona and I. hexagona hybrid sites. The two species did not differ in leaf production at the I. fulva and I. fulva hybrid sites. Leaf production by both classes of hybrid was as great as, or significantly greater than, both parental classes in all sites. Iris hexagona rhizomes gained mass in the I. hexagona and I. hexagona hybrid sites, but lost mass in the I. fulva and I. fulva hybrid sites. Iris fulva rhizomes lost mass in all sites. There were no significant differences in rhizome growth among classes at the I. fulva site. At all other sites, F₁ rhizomes grew significantly more than all other classes except for I. hexagona at the I. hexagona hybrid site. There were no significant differences among classes in the production of new ramets. Overall blooming frequencies were 30% for I. fulva, 10% for F₁s, 3% for F₂s, and 0.7% for I. hexagona. Blooming frequency did not differ among sites for I. fulva, but significantly more F₁s bloomed at the I. hexagona site than at the I. fulva site. These results are inconsistent with all three models of hybrid zone structure. They suggest that once rhizomes become established, hybrids can reproduce by clonal growth as successfully as parents in all habitats, and can outperform them in some habitats. Clonal reproduction may ensure the long-term survival of early generation hybrids and allow the establishment of introgressed populations, despite the fact that F₁ hybrids are rarely produced in nature.     

POLLEN-TUBE COMPETITION,SIRING SUCCESS,AND CONSISTENT ASYMMETRIC HYBRIDIZATION IN LOUISIANA IRISES

Postpollination mechanisms can play an important role in limiting natural hybridization in plants. Reciprocal hand pollination experiments were performed to study these mechanisms in two species of Louisiana iris: Iris brevicaulis and I. fulva. Relative pollen-tube growth rates changed significantly through time, with I. fulva tubes increasingly outperforming I. brevicaulis tubes in both conspecific and heterospecific styles. However, this pattern of change in relative performance was a poor predictor of siring success: the majority of seeds sired by both maternal species was conspecific rather than hybrid. Experimental crosses and field studies show consistent asymmetric hybridization in Louisiana irises, with I. fulva being a more successful father and a more selective mother than both I. brevicaulis and a third species, I. hexagona. The cause of this pattern is not yet clear, but the pattern itself is unusual. Typically, short-styled species tend to be less successful in reciprocal crosses than long-styled relatives, but I. fulva has shorter styles than either I. brevicaulis or I. hexagona. The effects of pollen-tube competition, differential fertilization, and selective abortion in causing this pattern of asymmetric hybridization is discussed.     

Frequency and spatial patterning of clonal reproduction in Louisiana iris hybrid populations

Abstract.— The plant genera in which natural hybridization is most prevalent tend to be outcrossing perennials with some mechanism for clonal (i.e., asexual) reproduction. Although clonal reproduction in fertile, sexually reproducing hybrid populations could have important evolutionary consequences, little attention has been paid to quantifying this parameter in such populations. In the present study, we examined the frequency and spatial patterning of clonal reproduction in two Louisiana iris hybrid populations. Allozyme analysis of both populations revealed relatively high levels of genotypic diversity. However, a considerable amount of clonality was apparent. Nearly half of all genets (47%) in one population and more than half (61%) in the other had multiple ramets. Furthermore, both populations exhibited relatively high levels of genetic structuring, a pattern that resulted from the aggregation of clonal ramets. The occurrence of clonal reproduction in hybrid populations could not only facilitate introgression through an increase in the number of flowering ramets per genet and/or the survivorship of early generation hybrids, but might also influence the mating system of such populations. Any potential increase in the selfing rate due to cross-pollination among ramets of the same genet may, in turn, increase the likelihood of homoploid hybrid speciation.     

Pollinator behaviour and the evolution of Louisiana iris hybrid zones

Pollinator preference may influence the origin and dynamics of plant hybrid zones. Natural hybrid populations between the red‐flowered Iris fulva and the blue‐flowered Iris brevicaulis are found in southern Louisiana. The genetic structure of these populations reflects a lack of intermediate genotypes. We observed pollinator behaviour in an experimental array with five plants each of I. fulva, I. brevicaulis, their F₁, and the first backcross generation in each direction, to obtain data on flower type preferences and transitions between flower types. The most abundant visitors were Ruby‐throated Hummingbirds (Archilochus colubris) and workers of the bumblebee Bombus pennsylvanicus. Hummingbirds visited I. fulva twice as often as I. brevicaulis and visited hybrids at intermediate frequencies. Bumblebee workers preferred the purple‐flowered F₁s and visited plants of I. fulva and the backcross to I. fulva more often than I. brevicaulis and its backcross. Overall, F₁ flowers were visited most frequently. Both hummingbirds and bumblebees visited nearest neighbours in almost 80% of the interplant movements. This meant that a majority of movements were between different flower types, rather than between plants of the same type. Findings from the present study suggest that pollinator preference is not a major causal factor for the lack of intermediate genotypes in natural iris hybrid populations. Instead, pollinator behaviour in our array promoted mixed mating between flower types belonging to different pollination syndromes. However, owing to predominant nearest‐neighbour visitation, the spatial distribution of parental and hybrid genotypes (in concert with pollinator behaviour) will have a strong influence on mating patterns and thus the genotypic structure and evolution of Louisiana iris hybrid zones.