Mexican Populus tremuloides Michx: Adaptation to climate change under extreme heterozygote excess

The genetic structure of Populus tremuloides Michx. populations in Mexico is largely unknown. A study of this species throughout its range is critical to determine the genetic signatures of its adaptation to different local environmental conditions due to long-term natural selection and its adaptability to progressive environmental alterations such as global climate change. Among few other genetic studies on Mexican P. tremuloides, a recent study analyzed the genetic diversity of 91 populations of P. tremuloides across its distribution in Mexico. After sequencing, 1,556,064 unfiltered SNPs were detected, while 36,810 filtered SNPs remained. The genetic diversity was explored using observed heterozygosity (H_o), expected heterozygosity (H_e), and inbreeding coefficient (F_IS). The results unexpectedly showed that the genetic diversity expressed by the expected heterozygosity is significantly smaller (mean H_e of 0.09, [0.05–0.11]) than that of the US and Canadian populations (mean H_e ≥ 0.14, depending on the study). The inbreeding coefficient of populations was approximately between 0 and − 1. This means that there was no inbreeding and that the observed heterozygosity is much higher than expected. Clonal richness was strongly positively associated with F_IS and H_e. The polyploidy degree was not correlated with F_IS, but positively correlated with H_o and H_e. Partial asexual reproduction in small populations, where genetic drift predominates over mutation, negatively affects H_e and F_IS values (caused by heterozygote excess). Negative assortative mating and progressive selection against homozygotes over the life cycle may also contribute to these unexpected and unique genetic relationships. Asexual reproduction of Mexican poplar may help it adapt to extreme ecological conditions such as severe droughts. While the Meselson effect caused by partial asexual reproduction does not provide a direct adaptation benefit for the Mexican populations of this poplar, it could indirectly support the survival of these very small and isolated populations by increasing the genetic diversity of individuals through extreme heterozygosity excess. Genetic diversity is important for the adaptation of populations, as it provides a pool of genetic variation that can be selected for or against in response to changing environmental conditions like climate change.