Pollen competition in hybridizing Cakile species: How does a latecomer win the race?

PREMISE: Hybridization between cross-compatible species depends on the extent of competition between alternative mates. Even if stigmatic compatibility allows for hybridization, hybridization requires the heterospecific pollen to be competitive. Here, we determined whether conspecific pollen has an advantage in the race to fertilize ovules and the potential handicap to be overcome by heterospecific pollen in invasive Cakile species. METHODS: We used fluorescence microscopy to measure pollen tube growth after conspecific and heterospecific hand-pollination treatments. We then determined siring success in the progeny relative to the timing of heterospecific pollen arrival on the stigma using CAPS markers. RESULTS: In the absence of pollen competition, pollination time and pollen recipient species had a significant effect on the ratio of pollen tube growth. In long-styled C. maritima (outcrosser), pollen tubes grew similarly in both directions. In short-styled C. edentula (selfer), conspecific and heterospecific pollen tubes grew differently. Cakile edentula pollen produced more pollen tubes, revealing the potential for a mating asymmetry whereby C. edentula pollen had an advantage relative to C. maritima. In the presence of pollen competition, siring success was equivalent when pollen deposition was synchronous. However, a moderate 1-h advantage in the timing of conspecific pollination resulted in almost complete assortative mating, while an equivalent delay in conspecific pollination resulted in substantial hybrid formation. CONCLUSIONS: Hybridization can aid the establishment of invasive species through the transfer of adaptive alleles from cross-compatible species, but also lead to extinction through demographic or genetic swamping. Time of pollen arrival on the stigma substantially affected hybridization rate, pointing to the importance of pollination timing in driving introgression and genetic swamping.

The tip of the iceberg: Genome wide marker analysis reveals hidden hybridization during invasion.

Biological invasions are accelerating, and invasive species can have large economic impacts as well as severe consequences for biodiversity. During invasions, species can interact, potentially resulting in hybridization. Here, we examined two Cakile species, C. edentula and C. maritima (Brassicaceae), that co-occur and may hybridize during range expansion in separate regions of the globe. Cakile edentula invaded each location first, while C. maritima established later, apparently replacing the former. We assessed the evidence for hybridization in western North America and Australia, where both species have been introduced, and identified source populations with 4561 SNPs using Genotype-by-Sequencing. Our results indicate that C. edentula in Australia originated from one region of eastern North America while in western North America it is probably from multiple sources. Cakile maritima in Australia is derived from at least two different parts of Europe while the introduction in western North America is from one. Although morphological evidence of hybridization is generally limited to mixed species populations in Australia and virtually absent elsewhere, our genetic analysis revealed relatively high levels of hybridization in Australia (58% hybrids using Admixture) and supported the presence of hybrids in western North America (16% hybrids using Admixture) and New Zealand. Hybrids might be commonly overlooked in invaders, as identification based solely on morphological traits may represent only the tip of the iceberg. Our study reveals a repeated pattern of invasion, hybridization and apparent replacement of one species by another, which offers an opportunity to investigate the role of hybridization and introgression during invasion.