Does selfing facilitate niche shifts? Support from niche modeling

Background/Question/Methods

Recent research found that plant species with high selfing rates have larger range sizes than outcrossers, likely due to higher colonization success via reproductive assurance in selfers. Theory suggests that high rates of selfing could hinder a species’ adaptability to novel habitats encountered during colonization, suggesting lower species-level environmental variation in the locations of selfing vs. outcrossing taxa (i.e. lower niche breadths in selfers). We examine the interrelationship between mating systems and species-level environmental variation to gain insights into processes that drive plant species’ distributions. Using paired selfing-outcrossing sister species, we explore three ideas: 1) that species pairs have evolved niche divergence 2) that selfers have narrower niche breadths than outcrossing species and 3) that species pairs express differences in physiology related to abiotic aspects of their niches. We applied species distribution modeling (SDM) to quantify environmental variation and estimate niche divergence, breadth, and overlap for seven sister pairs in Collinsia and Tonella (Plantaginaceae) from herbarium records. On a subset of species, we sampled leaf tissue from herbarium specimens and measured carbon isotope ratios, a drought stress metric. Since selfers inhabited sites of low precipitation relative to their outcrossing sisters, we expect δ¹³C values to be more negative in selfers.

Results/Conclusions

Overall, we found that selfing species are associated with greater environmental variation than outcrossing species. And, for six of the seven species pairs examined, selfers have significantly greater niche divergence than their outcrossing sister. and some degree of niche overlap with outcrossers. This suggests that selfers have evolved niches that are distinct from their outcrossing sisters. Selfers also had statistically greater niche breadth values than outcrossers. Multivariate analysis of environmental variables found that selfers were associated with a wider range of annual precipitation values than outcrossers, including more statistically extreme values. We are currently assessing δ¹³C values to determine if there are detectable differences in this metric between sister species. In summary, our SDM results challenge the long-held notion that selfing species are unable to adapt to novel environmental conditions. Instead, it supports a role for selfing in the capacity to reach, establish and adapt to new habitats, thus contributing to niche shifts.