Secondary origin, hybridization and sexual reproduction in a diploid-tetraploid contact zone of the facultatively apomictic orchid Zygopetalum mackayi

The production of triploids and apomictic reproduction are important processes for polyploid establishment and cytotype coexistence, but we know little about the interaction between triploids and facultatively apomictic plants. To bridge this gap, we studied the pollen-dependent, facultatively apomictic orchid Zygopetalum mackayi from high-elevation outcrops of southeast Brazil. We described the nature of the contact between Z. mackayi cytotypes and patterns of genetic diversity and structure based on eight microsatellite markers and 155 individuals of pure tetraploid, pure diploid and mixed cytotype populations. Our results revealed high values of genetic and genotypic diversity within all populations of Z. mackayi. Each cytotype emerged as a genetic distinct cluster, combining individuals from different populations. Triploids clustered in an intermediate position between diploids and tetraploids. Most genetic variance is associated with individuals within populations and genetic differentiation is high among populations. Mixed cytotype populations of Z. mackayi originate from secondary contact. Triploids are hybrids between diploids and tetraploids and likely act as a bridge. Our results point to the predominance of sexual reproduction in all populations but do not corroborate previous basic chromosome number for this species. Polyploidy rather than facultative apomixis may explain the larger geographic distribution of tetraploids of Z. mackayi. (AU)