When island-like populations at high elevation show genetic divergence despite no morphological variability: The case of Lupinus montanus in Central Mexico

Lupinus montanus subsp. montanus var. montanus Kunth is a widespread taxon occurring throughout the highlands of Central Mexico and Guatemala. Populations of this variety show little variation in plant morphology, but their highly disjunct islandlike distribution suggests that genetic differentiation between populations should be expected. To test this idea, we assessed genetic diversity among 13 populations of Lupinus montanus var. montanus growing on the 6 main volcanoes of the Trans-Mexican Volcanic Belt (TMVB) using 4 intersimple sequence repeat markers to quantify genetic divergence. The 93 loci identified showed significant differentiation between populations (F_ST ≈ 0.5) and high levels of genetic diversity among populations (P = 97.9%, He = 0.29, H = 0.44). A clear segregation between eastern and western populations was revealed. Among the eastern populations, we did not find a significant structure, but there was a trend towards a site-dependent effect, indicating very recent divergence. The results reveal in situ diversification events, an East-West split suggestive of older divergence, and more recent incomplete divergence among the eastern populations, which can be attributed to a combination of the impacts of Pleistocene glaciations and the geological history of formation of the TMVB volcanoes. Lack of morphological differentiation may be the product of recent isolation and stabilising selection.