Sosa, K. | bioRxiv Science

bioRxiv2024

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

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.

evolutionary biology↗

bioRxiv2024

Short steps can take you far: Phylogenetic analysis of Australasian Cheilanthes distans reveals frequent shorter-range dispersal

Biologists have long pondered species geographical distributions and sought to understand what factors drive dispersal and determine species ranges. In considering plant species with large ranges, a question that has remained underexplored is whether large ranges are attained primarily through many instances of short scale dispersal or whether instead widespread ranges are attained by propagules with increased dispersal distances. Ferns provide an ideal system to explore this question as their propagules are very small spores, which have been theorised can be carried by wind to essentially anywhere on the planet. Unfortunately, population-level genetic data in ferns is relatively uncommon, limiting our ability to answer this and related questions. For this work, I focus on Cheilanthes distans (Pteridaceae) as a study system, a widespread fern with extensive spore variation that occurs over Australia and into New Zealand/Aotearoa, New Caledonia, and other Pacific islands. I sampled widely across the species range, in addition to across Australasian Cheilanthes (as a robust tree for the genus does not exist), ultimately building a phylogeny based on the GoFlag 451 bait set. With these data, we can investigate additional questions, including whether reproductive mode, polyploidy, or lineage influence dispersal, as well as whether movement is occurring randomly or is instead asymmetrical. I explored the relationships between sexual and apomictic specimens to understand whether the former are the parental lineages to apomictic plants and whether we find evidence for apomictic plants dispersing out of a small parental range. I investigated how many times polyploid lineages have arisen in C. distans and whether they are each limited geographically, perhaps forming isolated ranges that collectively result in C. distans larger range. Additionally, I generated estimates for ancestral ranges and dispersal between populations to understand whether certain lineages are limited to particular geographic regions, to explore the directionality of dispersal, and to assess whether most movement is happening over short or long distances. Particularly interestingly, I find that most dispersal in this species appears to occur over smaller steps rather than longer jumps, underscoring how short movement can nevertheless allow for establishment of large ranges; this dispersal is not limited phylogenetically and seems to occur equally for all lineages. What is more, I find evidence for asymmetrical dispersal directionality, apparently most frequently tracking trade winds. These findings demonstrate the importance of population-level data, and provide concrete results that add nuance to long-standing dispersibility hypotheses in the fern community that have, until now, lacked empirical data.

evolutionary biology↗