The adaptive value of heterospory: Evidence from Selaginella

Abstract

Heterospory was a pivotal evolutionary innovation for land plants, but it has never been clear why it evolved. We used the geographic distributions of 114 species of the heterosporous lycophyte Selaginella to explore the functional ecology of microspore and megaspore size, traits that would be correlated with many aspects of a species' regeneration niche. We characterized habitats at a global scale using leaf area index (LAI), a measure of foliage density and thus shading, and net primary productivity (NPP), a measure of growth potential. Microspore size tends to decrease as habitat LAI and NPP increase, a trend that could be related to desiccation resistance or to filtration of wind-borne particles by leaf surfaces. Megaspore size tends to increase among species that inhabit regions of high LAI, but there is an important interaction with NPP. This geographical pattern suggests that larger megaspores provide an establishment advantage in shaded habitats, although in open habitats, where light is less limiting, higher productivity of the environment seems to give an advantage to species with smaller megaspores. These results support previous theoretical arguments that heterospory was originally an adaptation to the increasing height and density of Devonian vegetative canopies that accompanied the diversification of vascular plants with leaves.

Nuclear Genome Size is Positively Correlated with Mean LTR Insertion Date in Fern and Lycophyte Genomes

“…The heterosporous Selaginella and the two aquatic ferns Azolla filiculoides and Salvinia cucullata share similar LTR dynamics characterized by a predominance of recent insertions. Although there may be several adaptive roles of heterospory in vascular plants (Petersen and Burd 2018), it does function to promote outcrossing. In contrast, LTR dynamics in the homosporous ferns which are capable of a greater array of selfing mechanisms, have relatively fewer recent insertions.…”

The role of spore size in the global pattern of co‐occurrence among Selaginella species

“…Within a common range (indeed, within the same community), species could specialize on different regeneration microsites. For example, species with large megaspores may successfully establish in deep shade that tends to exclude species with small megapores (as the findings of Petersen & Burd, suggest). Yet small megaspores may succeed in exposed microsites on rocky outcrops, steep slopes and disturbed riparian zones, which they “find” more readily through dispersal than do larger megaspores.…”

“…A variety of ecological and evolutionary processes could affect biogeographical patterns of co‐occurrence (McGill, Enquist, Weiher, & Westoby, ). It is known that trends in megaspore and microspore sizes among Selaginella species are related to the foliage density and net primary productivity of their habitats (Petersen & Burd, ). Thus, the larger the region of sympatry between two species, the more they are likely to share similar environments and the more similar we might expect their spore sizes to be.…”

“…Heterospory arose independently in several land plant lineages, and its evolution is analogous in many ways to anisogamy, the difference in resource investment that defines female and male gametes (Petersen & Burd, ). Heterospory was an essential precursor to the evolution of the seed habit (Willis & McElwain, ), and some ecological parallels between megaspore size in Selaginella and seed size among spermatophytes have already been documented (Petersen & Burd, ).…”

“…Microspore size tends to decline as either foliage density or primary productivity increases, probably because numerical abundance is an advantage when foliar surfaces filter wind‐borne microspores. Spore size seems, therefore, to be an ecologically sensitive trait that could provide the basis for niche differentiation between species (Petersen & Burd, ).…”