Abstract
Hybridization has the potential to contribute to phenotypic and genetic variation and can be a major evolutionary mechanism. However, when hybridization is extensive it can also lead to the blurring of species boundaries and the emergence of cryptic species (i.e., two or more species not distinguishable morphologically). In this study, we address this hypothesis in Epidendrum, the largest Neotropical genus of orchids where hybridization is apparently so common that it may explain the high levels of morphological diversity found. Nonetheless, this hypothesis is mostly based on the intermediacy of morphological characters and intermediacy by itself is not a proof of hybridization. Therefore, in this study, we first assessed the existence of hybrids using cpDNA and AFLP data gathered from a large-scale sampling comprising 1038 plants of three species of Epidendrum (E. calanthum, E. cochlidium and E. schistochilum). Subsequently, a Bayesian assignment of individuals into different genetic classes (pure species, F1, F2 or backcross generations) revealed that hybrid genotypes were prevalent in all sympatric populations. In most cases, parental species were not assigned as pure individuals, rather consisting in backcrossed genotypes or F1 hybrids. We also found that reproductive barriers are apparently very weak in Epidendrum because the three species largely overlapped in their flowering periods and interspecific crosses always produced viable seeds. Further, hybridization contributed to enhance floral variability, genome size and reproductive success since we found that these traits were always higher in hybrid classes (F1, F2 and backcrosses) than in pure parental species, and offer an explanation for the blurring of species boundaries in this genus of orchids. We hypothesize that these natural hybrids possess an evolutionary advantage, which may explain the high rates of cryptic species observed in this genus.
Highlights
The role of hybridization in plant speciation is still one of the most exciting issues in evolutionary biology [1]
Genetic composition of the hybrids Based on amplified fragment length polymorphism (AFLP) data, Bayesian assignment of individuals indicated that the allopatric populations of E. calanthum, E. cochlidium and E. schistochilum were generally composed of purebreds since only one individual (E. schistochilum) was assigned as F1 genotype (Figure 1)
Backcrosses with E. calanthum were predominant in the sympatric populations of E. calanthum and E. cochlidium (54.7% of individuals), while backcrosses with E. schistochilum were abundant when it occurred in sympatry with E. calanthum (58.1% of individuals; Figure 1)
Summary
The role of hybridization in plant speciation is still one of the most exciting issues in evolutionary biology [1]. Speciation in plants via hybridization is apparently more common than previously thought with several studies suggesting that at least 40% of plant species may have arisen from this process [2,3,4,5,6,7] In addition to this constructive role, where hybridization may give rise to new lineages, introgression of genes may increase genetic variation [8] and genome size [9], but may lead to the genetic assimilation of their congeners [10]. (Orchidaceae), molecular studies confirming this hypothesis are scarce [11] It is the largest neotropical genus of Orchidaceae with almost 1500 species described [12] and hybridization has been suggested to explain the high levels of morphological diversity observed and the existence of cryptic species (i.e., two or more species not distinguishable morphologically). The rise of relatively fast DNA sequencing techniques has given biologists the power to differentiate among morphologically similar entities [15,16,17]
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