The significance of chlorophyllous spores in the evolution and biogeography of ferns

Ferns, the second most diverse group of vascular plants globally, have evolved over millions of years colonizing diverse habitats across almost all continents. Ferns possess a suit of morphological and physiological traits that have enabled them to thrive in multiple habitats. A particular trait, is the presence/absence of chlorophyll in spores, which may constitute a bottleneck in the establishment of new populations due to the physiological differences between both spore types. Chlorophyllous spores (hereafter CS) are present in approximately 15% of fern species, and about 70% of them are epiphytes. Unlike non-chlorophyllous spores (hereafter non-CS), CS have a shorter viability, lack dormancy, and often have a thinner perispore. Despite these differences, CS have received little attention, leaving several unanswered questions regarding their role in fern ecology. To address this gap, here I investigate the influence of spore type on the evolution and distribution of ferns using phylogenetic and biogeographical approaches. First, I evaluate the taxonomic distribution of spore type in all extant genera of ferns by compiling a database of chlorophyll presence or absence in spores through a systematic literature review. Furthermore, using phylogenetic comparative methods, I test the hypothesis of whether CS in ferns are a compensation for habitats that are poor in mycorrhizal associations. I investigate the correlated evolution between spore type and habits (i.e. epiphytic and terrestrial) and analyze how the presence of CS influences the evolutionary transitional rates toward these habits. Additionally, I document the global distribution of chlorophyllous-spored species (hereafter CSS). Specifically, I correlate their species richness proportions with environmental variables. Finally, I explore how the phylogenetic relationships drive their latitudinal distribution patterns. My findings reveal that spore type coevolved with the habits used by ferns. Although I found weak evidence for the coevolution of CS and habitats poor in mycorrhizae, I showed that CSS have more transitions from terrestrial to epiphytic habits than non-chlorophyllous spored species. Furthermore, I found that CSS are overrepresented on islands globally, and that their richness proportions peak in higher latitudes with marked patterns for epiphytic CSS. Additionally, I found phylogenetic evidence suggesting that their distribution is also driven by phylogenetic niche conservatism. Overall, this thesis contributes to unraveling the often neglected role of CS in the evolution and global distribution of ferns. These findings evidence that, for better understanding of evolutionary and distributional patterns, is key to consider the effects of functional traits, integrating ecological and phylogenetic perspectives.

2025-05-22