Hybrid fertility and the rarity of homoploid hybrid speciation.

Hybrid speciation is increasingly being recognized as an important driver in diversification. Of the two forms of hybrid speciation, polyploid hybrid speciation is considered more common as it provides instant reproductive isolation either completely or partially, due to meiotic incompatibilities with the parental species. Homoploid hybrid speciation is considered rare, as it lacks the instant reproductive isolation conferred by polyploidy, though there are an increasing number of examples in the literature. Reproductive isolation in the nascent homoploid hybrid species is commonly achieved by niche divergence, often into territory with one or more forms of abiotic stress unsuitable for parental species. However, reproductive isolation may be due to factors beyond mere niche divergence. In this paper, I examine the pollen fertility of hybrids compared with their parental species. Using a dataset of over 2000 observations, I compared F1 and F2 hybrids to members of the same genus and family. F1 hybrids universally have lower mean fertility than good species in their group at both the genus and family levels, and most of those differences are significant. Data for F2 hybrids are limited and conflicting, but there may be a path to restored fertility across multiple generations such as through fertility restoration genes or selection for increased fertility. Alternatively, homoploid hybrids may rely on forms of asexual reproduction. Examples exist to support both alternatives. While pollen fertility is a useful metric, other metrics of plant fertility could also throw light on the difficulty of forming a homoploid hybrid species in plants.