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

Abstract

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.

Asymmetric Reproductive Barriers and Gene Flow Promote the Rise of a Stable Hybrid Zone in the Mediterranean High Mountain

“…This is supported in our case by the phenotypic stability of the contact zone after 10 years, and it is especially relevant because it makes the scenarios of non-sister species and non-completed speciation compatible as the origin of this hybrid zone. These hybridizing contact zones have important effects on the evolutionary dynamics of the hybridizing species and the biodiversity in the area where the contact happens (Wielstra, 2019;Alves de Moura et al, 2020). The hybridization is not always evident for human, which could have even more pervasive consequences for the biology of the species and their conservation (Gómez et al, 2015;Waldron et al, 2019).…”

Apomixis beyond trees in the Brazilian savanna: new insights from the orchid Zygopetalum mackayi

“…Given recent studies on cytogenetics, genetics, reproductive biology, and ecological niche modelling of Z. mackayi (Campacci et al 2017;Gomes et al 2018;Moura et al 2020) and the inconsistency of previous studies on the developmental mechanism of apomixis (Suessenguth 1923;Afzelius 1959), we here clarify the relationship among apomixis, chromosome numbers and polyembryony in this species. Speci cally,…”

“…The distinction between LSI and inbreeding depression is challenging, but LSI mechanisms are known to cause fruit abortion at the same developmental stage for a given species, while inbreeding depression acts in different stages of a plant´s life cycle, mainly along embryo development (Seavey and Bawa 1986;Gibbs 2014). In addition, population genetic studies of Z. mackayi discarded the occurrence of inbreeding in any cytotype (Moura et al 2020).…”

“…Later, Gomes et al (2018) showed Z. mackayi exhibits three different cytotypes (diploid, 2N = 48; triploid, 2N = 72; and tetraploid, 2N = 96). Diploids and tetraploids are geographically structured and associated with different climatic conditions, while triploids are F1 hybrids that occur in a contact zone where diploids and tetraploids meet (Gomes et al 2018;Moura et al 2020). In addition, studies of reproductive biology (Campacci et al 2017) and population genetics (Moura et al, 2020) suggested Z. mackayi reproduces mostly sexually and facultatively by apomixis, being the embryo formation pollination-dependent for fruit development (Campacci et al 2017).…”

Triploid cultivars of Cymbidium act as a bridge in the formation of polyploid plants

“…Polyploidy plays an important role during the evolution of orchids as sequence analyses showed that wholegenome duplication (WGD) occurred widely in orchids, including Apostasia shenzhenica Zhang et al, 2021), Cymbidium ensifolium (Ai et al, 2021), Dendrobium chrysotoxum (Zhang et al, 2021), and Phalaenopsis (Cai et al, 2015). A recent study showed that triploids clustered in an intermediate position between diploids and tetraploids in Zygopetalum mackayi (Moura et al, 2020). In the Nigritella nigra group, nuclear and plastid marker analysis showed that tetraploid N. nigra subsp.…”