The importance of mistakes: Variation in spores reveals trade-offs for range expansion in the xeric-adapted Australasian species Cheilanthes distans (Pteridaceae)

Abstract

Biological trade-offs present a central issue for evolutionary biology: it has been a fundamental understanding within the field that limits exist on the phenotypic traits a species is able to exhibit in part due to trade-offs. Reproduction--with its myriad forms--has been studied extensively in the context of these dynamics. And while considerable literature has explored trade-offs between seed size and number and their associated environmental conditions, none has looked at spore size trade-offs in ferns. We can hypothesise potential trade-offs in spore size: smaller spores should be able to disperse farther, but may not have sufficient provisions to survive in environments that require them to remain at the gametophyte stage for longer periods if their germination cues are mismatched. Reproductive mode (sexual vs. asexual) and ploidy may also be playing a role. In order to study trade-offs related to spore size, I focus on the Australasian fern species Cheilanthes distans (Pteridaceae), which is most often found in xeric environments, growing in crevices or on top of rocks which are haphazardly scattered across their range. Apomictic diplospores in this species are formed through first division restitution, a meiotic pathway particularly prone to mistakes in chromosome pairing and cell division (as compared to premeiotic endomitosis). Rather than being problematic, these mistakes ultimately lead to considerable additional variation in spore size, spore products (through a range of aneuploid spores), and spore ploidy. In this study, I explore trade-offs between spore size, dispersal, and germination, taking into account effects from reproductive mode and ploidy. I carried out an extensive survey of C. distans specimens to establish the prevalence of sexual vs. apomictic (asexual) specimens, and to describe in greater depth the variation in ploidy across the species. I also collected data on spore size and sporogenesis forms. With these data I then asked: is spore size correlated with range area or with germination? And does spore form correlate with either spore size or germination? Ultimately, I find that variations in sporogenesis may be leading to large variation in spore sizes--especially since spores traditionally considered abortive are in fact viable--and that this variation may provide abundant fodder for evolution to act through trade-offs between dispersal into large ranges and germination leading to establishment. Especially in light of the fact that many spores that were historically considered abortive are fully viable and likely shaping evolution in important ways, it is worth remarking on what these results illustrate more broadly: the way in which we have constructed disability ultimately affects how we perceive so-called genetic errors--both in humans and in other species--and thus limits what we allow ourselves to imagine disabled beings are capable of.