Causes of heterozygosity excess: The case of Mexican populations of Populus tremuloides

The presence of heterozygous individuals in a population is crucial for maintaining genetic diversity, which can positively affect fitness and adaptability to environmental changes. While inbreeding generally reduces the proportion of heterozygous individuals in a population, polyploidy tends to increase the proportion. North American <i>Populus tremuloides</i> is one of the most widely distributed and ecologically important tree species in the Northern Hemisphere. However, genetic variation in Mexican populations of <i>P. tremuloides</i>, including the genetic signatures of their adaptation to a variety of environments, remains largely uncharacterized. The aim of this study was to analyze how inbreeding coefficient (<i>F</i> _IS) and ploidy are associated with clonal richness, population cover, climate and soil traits in 91 marginal to small, isolated populations of this tree species throughout its entire distribution in Mexico. Genetic variables were determined using 36,810 filtered SNPs derived from genome re-sequencing. We found that <i>F</i> _IS was approximately between 0 and -1, indicating an extreme heterozygosity excess. One key contributor to the observed extreme heterozygosity excess was asexual reproduction, although ploidy levels cannot explain this excess. Analysis of all neutral SNPs showed that asexual reproduction was positively correlated with observed heterozygosity (<i>H</i> _o) but negatively correlated with expected heterozygosity (<i>H</i> _e). Analysis of outlier SNPs also showed that asexual reproduction was positively correlated with <i>H</i> _o and negatively correlated with <i>H</i> _e, although this latter correlation was not significant. These findings support the presence of a Meselson effect.