The Journal of Heredity 88(6):545-549 (Nov-Dec 1997).
Differences in pollen-tube growth rate and reproductive isolation between Louisiana irises
S. E. Carney and M. L. Arnold

Most pollinations of I. fulva are performed by hummingbirds, while I. brevicaulis is most frequently pollinated by bumblebees (Viosca 1935). However, studies of mating patterns in these species demonstrate that interspecific visits are relatively common (Cruzan and Arnold 1994). The possibility of reproductive isolation due to pollen-tube growth phenomena is discussed below. Additionally some hybrid genotypes abort at a higher frequency than do parental genotypes (Cruzan and Arnold 1994). While none of these mechanisms provides complete isolation, they act in concert to limit hybridization between these species. However, barriers to hybridization between Louisiana iris species are reduced in later generations (e.g., F2, BC1; Hodges et al. 1996).

A level of reproductive isolation is necessary for the process of genetic divergence. Such isolation also prevents the homogenization of species following secondary contact. This study is an investigation of the relative contribution of two prefertilization mechanisms, pollen-tube growth rate and pollen-tube attrition, to the reproductive isolation of Iris fulva and I. brevicaulis, two naturally hybridizing members of the Louisiana iris species complex. Flowers of each species were first pollinated with heterospecific pollen. After various time intervals, conspecific pollen was added. Analyses of the patterns of resulting progeny were used to infer whether relative pollen-tube growth rates act as a prefertilization isolating mechanism. In I. fulva the frequency of hybrid seeds increased with increasing pollination interval, suggesting that hybridization is limited by pollen-tube growth rates. Likewise, in I. brevicaulis hybrid seed production increased, but it was high regardless of the pollination interval. Thus it appears that relative pollen-tube growth rates limit interspecific reproduction in both species, but barriers are weaker in I. brevicaulis.