Homosporous versus Heterosporous pteridophytes and genetic diversity?

Homosporous plants produce potentially hermaphroditic gametophytes and are therefore capable of producing an allotetraploid sporophyte from one spore, which may confer an advantage in escaping hybrid sterility relative to heterosporous plants. This process, repeated, has produced mean genome sizes in homosporous plants that are considerably larger than found heterosporous plants. Lineages predisposed to polyploid escape from hybrid sterility may experience at least two selective advantages, including: (1) an increased capacity for species production and (2) an increased capacity for species persistence associated with increased (allopolyploid) phenotypic plasticity. These advantages may explain, in part, the success of some homosporous lineages, particularly within leptosporangiate ferns. Alternatively, genome size and species richness may increase due to an irreversible “ratchet effect” associated with polyploidy that can persist despite deleterious effects. Chromosome number and species richness data was compiled from the literature for 40 genera and 1,174 species of homosporous lycophytes and monilophytes and superimposed on the most recently published phyologenies. Analyses conducted for a number of taxonomic grades found a positive correlation between genome size and clade age; however, no relationships were observed between species richness and genome size or species richness and lineage age, globally or among sister-clades. Thus, the large genome sizes of homosporous plants appear to be the result of genome “ratcheting” and do not appear to reflect an allopolyploid advantage to species production or persistence.