The genetic and demographic consequences of hybridization in small plant populations
Hybridization is widely believed to be a homogenizing force and a source of genotypic novelty. However, its role in causing the local extinction of small plant populations is not well understood. I assessed the magnitude and direction of hybridization between small populations of red mulberry (' Morus rubra' L.) and its more abundant congener, white mulberry (' Morus alba' L.) in Canada and measured its effect on conspecific mating and establishment. Molecular analysis using nuclear markers (Randomly Amplified Polymorphic DNA) showed that hybrid frequency was relatively high (53.7%). Most hybrids (67%) had proportionately more white than red mulberry markers in their nuclear genome and the degree of white-bias increased with increasing population frequency of white mulberry. Furthermore, 68% of hybrids had the same chloroplast genome as white mulberry. These results indicate that introgression is bidirectional but asymmetrical and is related, in part, to the relative frequency of parental taxa. Hybridization also affected conspecific mating in red mulberry, where experimental removal of whites and hybrids increased the relative siring fertility of red mulberry by 14%, but there was no change in seed set. This indicates that ovules of red mulberry are discounted by heterospecific pollen, most of which is generated by hybrid plants. Moreover, pollen production in red mulberry was lower (2.7% to 10.3% of the total) than the mean of white and hybrid mulberry combined. Collectively, these results suggest that red mulberry is experiencing a mating disadvantage associated with low abundance. The effect on seed production is exacerbated at establishment, where reciprocal transplants of red mulberry were not ecologically differentiated and were consistently less fit than white and hybrid mulberry in all environments. Common garden experiments revealed that variation in offspring fitness was determined largely by the identity of the maternal parent; specifically, progeny with white mothers had the highest fitness. These results indicate that red mulberry is at a strong establishment disadvantage. Given that hybridization with white and hybrid mulberry has a negative effect on the formation and establishment of red mulberry, it is likely that asymmetrical gene flow will lead to the genetic assimilation of red mulberry in southern Ontario.