Mating system variation in hybridizing irises: Effects of phenology and floral densities on family outcrossing rates

Abstract

The frequency of outcrossing in two hybridizing species of Iris was estimated for populations and for individual fruits. Effects of floral phenology and the local densities of flowers on outcrossing rates were examined and the potential for hybrid seed formation under different pollen environments was assessed. The populations examined differed with respect to the spatial distribution of plants and the level of genetic structure; the I. fulva population consisted of a number of low density patches and appeared to have some genetic differentiation whereas the I. hexagona population consisted of a single high-density experimental plot of randomly distributed genotypes. Population outcrossing rate estimates were relatively high (0.67-0.90) for both species. The distribution of family outcrossing rates tended to be bimodal for both species with individual fruits either having the majority of seeds fertilized by outcrossed donors or being almost entirely selfed. fulva may have been a consequence of higher levels of vegetative reproduction and genetic structure and the behaviour of pollen vectors. Differences in pollen prepotency and the higher selfing rates observed at low floral densities in I. hexagona may have contributed to the observed patterns of hybrid seed formation. The analyses of family outcrossing rates provide important information on factors responsible for mating system variation and evolution.

The influence of population density on outcrossing rates in Mimulus ringens

“…Complete paternity exclusion provides more precise estimates of individual outcrossing rates than have been possible with multilocus estimation procedures (Hamrick & Schnabel, 1985;Brown et al, 1989;Murawski & Hamrick, 1992;Cruzan et a!., 1994). Outcrossing rates of pollen-fertile individuals varied considerably within populations, ranging from 0.10 to 0.55 in one of the high-density populations.…”

Loci Affecting Long-Term Hybrid Survivorship in Louisiana Irises: Implications for Reproductive Isolation and Introgression

“…In locally sympatric populations, a number of prezygotic reproductive isolating mechanisms strongly limit gene flow. Divergent flowering phenologies may prevent the initial production of F 1 hybrids, with I. fulva initiating flowering about a month earlier than I. brevicaulis in southern Louisiana populations (Cruzan and Arnold 1994). Habitat isolation also exists, with I. fulva preferring wet, periodically flooded woodlands, while I. brevicaulis is often found in drier habitats (Cruzan and Arnold 1993;Johnston et al 2001).…”

“…Iris brevicaulis, on the other hand, is mostly pollinated by bumblebee vectors, and has a suite of characters presumably adapted for attracting bee pollinators, including blue petals and sepals with white and yellow nectar guides, recessed anthers, and sturdy flower components that serve as landing pads for bumblebees . Some amount of postzygotic isolation is also observed between these two species in the form of reduced hybrid viability and fertility (Cruzan and Arnold 1994;Burke et al 1998;Bouck 2004). Despite the presence of multiple pre-and postzygotic barriers, these species form hybrid zones where they coexist, and extensive introgression has been documented in natural populations Cruzan and Arnold 1993;Johnston et al 2001).…”

“…These long-lived perennials reproduce both sexually and clonally, and both species have a mixed mating system, with large, showy flowers that reveal differing pollination syndromes (Viosca 1935;Cruzan and Arnold 1994;Cruzan et al 1994;Wesselingh and Arnold 2000). Both allopatric and sympatric populations can be encountered throughout the ranges of the species.…”

Outcrossing rates of individual Mimulus ringens genets are correlated with anther–stigma separation

“…Outcrossing rates of individual genets have been difficult to quantify because (1) some of the assumptions of the mixed-mating model, especially uniform spatial distribution of pollen allele frequencies, may be violated (Brown, 1990;Morgan & Barrett, 1990;Murawski & Hamrick, 1992;Cruzan et a!., 1994) and (2) standard errors for individual outcrossing rate estimates based upon the multilocus outcrossing estimation procedure (Ritland & Jam, 1981;Ritland, 1990) tend to be fairly large (Ritland & Ganders, 1985;Ritland, 1990;Boshier et at., 1995). Complete paternity analysis provides a more precise measure of individual outcrossing rates because every seed sampled from a maternal plant may be classified as either self or outcross (Brown et a!., 1989;Karron et at., 1995a).…”