Secondary origin, hybridization and sexual reproduction in a diploid- tetraploid contact zone of the facultative apomictic orchidZygopetalum mackayi

Abstract

>Mixed-cytotype populations are ideal to understand polyploid establishment and diversification. We used the orchid Zygopetalum mackayi to understand how facultative apomictic reproduction relates to polyploidy. Sexual diploids and facultative apomictic tetraploids occur under distinct niches, with a contact zone where triploids occur. We hypothesized that facultative apomictic reproduction increases the fitness of tetraploids through reproductive interference between cytotypes. We predict patterns of genetic diversity of allopatric tetraploid populations to be significantly different from contact zone populations as a result of dominant apomictic reproduction in the later. We also describe the contact nature of diploids and tetraploids and the role of the intermediate triploids based on patterns of genetic structure within and among pure and mixed-cytotype populations.>We designed eight microsatellite markers and genotyped 155 individuals from six populations resulting in 237 alleles. We described patterns of genetic diversity and structure within and among populations and cytotypes. >Genotypic diversity is similarly high among all populations and cytotypes. Each cytotype emerged as a genetically cluster, combining individuals from different populations. Triploids clustered in an intermediate position between diploids andtetraploids. >We rejected the hypothesis of reproductive interference between cytotypes of Z. mackayi. Patterns of genetic diversity are incongruent with the occurrence of apomict reproduction in tetraploids. Mixed-cytotype populations originate from secondary contact and triploids are hybrids between diploids and tetraploids and act as a reproductive barrier. We suggest polyploidy rather than facultative apomixis explains higher fitness of tetraploids in this species and, therefore, ecogeographical patterns of distribution.