Selfing species exhibit diminished niche breadth over time.

Abstract

Self-pollinating plants (\"selfers\") have larger geographic ranges and inhabit higher latitudes than their outcrossing relatives. This finding has led to the hypothesis that selfers also have broader climatic niches. It is possible that the increased likelihood of successful colonization into new areas and the initial purging of deleterious mutations may offset selfers inability to adapt to new environments due to low heterozygosity. Here, for the first time, we examine the climatic niches and mutation accumulation rates of hundreds of closely related selfing and outcrossing species. Contrary to expectations, selfers do not have wider climatic niche breadths than their outcrossing sister taxa despite selfers greatly expanded geographic ranges. Selfing sister pairs also exhibit greater niche overlap than outcrossing sisters, implying that climatic niche expansion becomes limited following the transition to selfing. Further, the niche breadth of selfers is predicted to decrease significantly faster than that of closely-related outcrossers. In support of these findings, selfers also display significantly higher mutation accumulation rates than their outcrossing sisters, implying decreased heterozygosity, effective population size, and adaptive potential. These results collectively suggest that while the release from mate limitation among selfing species may result in initial range expansion, range size and niche breadth are decoupled, and the limitations of an increasingly homogeneous genome will constrict selfers climatic niches and over time reduce their geographic ranges.