Understanding the geographic distributions of apomictic plants: a case for a pluralistic approach

Asexual organisms usually have larger, and in the Northern Hemisphere, more northern distributions than their sexual relatives. This phenomenon, called geographical parthenogenesis, has been attributed to predispositions in certain taxa, advantages of polyploidy and/or hybrid origin, advantages of uniparental reproduction, introgression of apomixis into sexuals, niche differentiation of clones, and biotic interactions. Here we focus on the role of uniparental reproduction in colonisation, and the importance of different developmental pathways, i.e. autonomous apomixis which does not require pollination and fertilisation of endosperm nuclei for successful seed set, and pseudogamous apomixis which does. A literature survey suggests that geographical parthenogenesis occurs frequently in species with autonomous apomixis, while the correlation with pseudogamy is poorly documented. However, taxonomic patterns (e. g. predominance of Asteraceae) and also methodological bias may influence estimates of frequencies of geographical parthenogenesis. We demonstrate that a flow cytometric seed screen (FCSS) is a powerful method for assessing pseudogamous vs. autonomous apomixis. We show that population genetic studies provide insights into the genetic diversity of apomicts, but do not give strong support for uniparental reproduction being the only explanation of geographical parthenogenesis. Molecular studies help elucidate the evolutionary and biogeographical history of apomictic complexes, and we conclude that multidisciplinary studies are needed to understand fully the phenomenon of geographical parthenogenesis.