Hybridisation as potential source of incongruence in the morphological and mitochondrial diversity of a Thai freshwater gastropod (Pachychilidae, Brotia H. Adams, 1866)

Abstract

Conflicts between morphology‐based taxonomy and mitochondrial phylogenies are a rampant phenomenon among cerithioidean freshwater gastropods, which is yet not completely understood. We have investigated snail populations sampled from several parts of the Moei River drainage in central Thailand that belong to a monophyletic mtDNA clade formally known as Brotia pagodula and recovered large degrees of variation in both the shell and mitochondrial sequences. When assessed independently, the levels of morphological and mitochondrial differentiation were indicative of the existence of more than one distinct species. Analyses of the shell resulted in the identification of two distinct morphotypes with a conical or elongate shape, respectively. Analyses of partial sequences of the mitochondrial genes 16S and COI revealed the existence of up to five well differentiated haplotype lineages. Rampant mismatch is observed when morphotypes are mapped onto the mtDNA phylogeny. Several explanations for this observation are discussed, such as the presence of paralogous sequences, incomplete lineage sorting of ancestral polymorphisms, presence of morphologically cryptic species and introgression of foreign haplotypes through hybridisation. The presence of nuclear gene copies of mitochondrial genes is refuted and incomplete lineage sorting at levels of up to 12% sequence divergence in COI in potentially panmictic populations is considered highly unlikely. We conclude that the discordance between morphologic and phylogenetic patterns are best explained by introgression of mtDNA haplotypes. Since Southeast Asian Pachychilidae are known to be poor dispersers and speciation is primarily thought to occur in allopatry, tectonic events and river captures during the Cenozoic are discussed as a potential factors mediating secondary contact between primarily allopatric sister species. We hypothesise that introgressive hybridisation between closely related species due to secondary contact has the potential to explain the mismatch between mitochondrial phylogenies and morphology‐based species trees found in many groups of cerithioidean freshwater gastropods. This hypothesis requires further testing by future studies. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Trachemys grayi emolli(Testudines, Emydidae) in Honduras and its systematic relationships based on mitochondrial DNA

“…Whereas mitochondrial DNA is particularly prone to introgression through hybridisation, a growing body of evidence suggest that introgressive hybridisation occurs frequently in the animal kingdom. Introgression of haplotypes may occur when species with incomplete mechanisms of reproductive isolation get into secondary contact with each other (e.g., Gutsche & Kaehler 2008;Kaehler & Deein 2010) and therefore animals held in captivity are particularly prone to hybridize even across species boundaries (e.g. Johnsgard 1960;Fong & Chen 2010).…”

Towards Solving Darwin's “mystery”: Speciation and Radiation in Lacustrine and Riverine Freshwater Gastropods*

“…comm., 2008), allowing only poor inference regarding the actual evolutionary relationship between species. I will hint at only a few examples where this holds true in freshwater gastropods (e.g., Walther et al 2006, Sengupta et al 2009) and among the Cerithioidea in Pachychilidae (Rintelen and Glaubrecht 2002, Köhler and Dames 2009, Köhler and Deein 2010, Rintelen et al 2010) and in Pleuroce ridae (e.g., Dillon and Frankis 2004, Lee et al 2007, Dillon and Robinson 2009). Nevertheless, molecular species-level phylogenies and the comparative method still allow certain insights into the origin of diversity by speciation and radiation, albeit mostly without explicit inference of the exact mechanisms.…”

“…An overview for hybridization is given in Coyne and Orr (2004), and recent discussion can be found in Seehausen (2004) and Mallet (2005), with case studies discussed in Glaubrecht (2010a). Using freshwater pachychilid gastropods of the genus Brotia H. Adams, 1866 from Thailand as examples of confl ict between shell morphology and genetic diversity based on mitochondrial gene fragments (16S and COI) forming well differentiated haplotype lineages, Köhler and Deein (2010) have discussed evidence for introgression of mtDNA haplotypes, and considered hybridization between closely related species due to secondary contact the most likely cause.…”

The role of barriers and gradients in differentiation processes of pyrgulinid microgastropods of Lake Ohrid

“…Introgressive hybridization (backcrossing of hybrids into parental populations) has been discussed as a potential cause of incongruence between phenotype and genotype (e.g. Funk & Omland, 2003; also see Köhler & Deein, 2010;von Rintelen et al, 2010 for examples in gastropods). Acknowledging that only a limited number of specimens have been studied, they all could easily be assigned to the respective depth forms based on morphological characters, and intermediate phenotypes are completely missing.…”