Genome size and climate segregation suggest distinct colonization histories of an orchid species from Neotropical high-elevation rocky complexes

Knowledge about the geographical distribution of cytotypes is crucial to understand the role of polyploidy in diversification. High-elevation rocky complexes (HERCs) are heterogeneous formations found in elevated areas of eastern Brazil. They harbour one of the most endemic floras within the Neotropical region. Yet, we lack knowledge concerning the association of environmental variables and plant cytotypes in this region. Here, we investigate whether the frequency of Zygopetalum mackayi orchid cytotypes is related to climate conditions in the HERCs. We describe chromosome counts, genome size estimates and their association with climate variables for 432 individuals from 19 localities. We show, for the first time, a strong association between climate variation and cytotype variation in a species from the HERCs. We confirm two cytotypes for Z. mackayi (2n = 48 and 2n = 96), which are geographically structured, and describe an intermediate cytotype (2n = 72) restricted to a contact zone. We discuss the implications of our results for chromosome evolution in this species and provide hypotheses for the origin and maintenance of cytotypes. (AU)