Abstract
BackgroundDNA barcoding has demonstrated that many discrete phenotypes are in fact genetically distinct (pseudo)cryptic species. Genetically identical, isogenic individuals, however, can also express similarly different phenotypes in response to a trigger condition, e.g. in the environment. This alternative explanation to cryptic speciation often remains untested because it requires considerable effort to reject the hypothesis that the observed underlying genetic homogeneity of the different phenotypes may be trivially caused by too slowly evolving molecular markers.The widespread squat lobster Munida gregaria comprises two discrete ecotypes, gregaria s. str. and subrugosa, which were long regarded as different species due to marked differences in morphological, ecological and behavioral traits. We studied the morphometry and genetics of M. gregaria s. l. and tested (1) whether the phenotypic differences remain stable after continental-scale sampling and inclusion of different life stages, (2) and whether each phenotype is underpinned by a specific genotype.ResultsA total number of 219 gregaria s. str. and subrugosa individuals from 25 stations encompassing almost entire range in South America were included in morphological and genetic analyses using nine unlinked hypervariable microsatellites and new COI sequences. Results from the PCA and using discriminant functions demonstrated that the morphology of the two forms remains discrete. The mitochondrial data showed a shallow, star-like haplotype network and complete overlap of genetic distances within and among ecotypes. Coalescent-based species delimitation methods, PTP and GMYC, coherently suggested that haplotypes of both ecotypes forms a single species. Although all microsatellite markers possess sufficient genetic variation, AMOVA, PCoA and Bayesian clustering approaches revealed no genetic clusters corresponding to ecotypes or geographic units across the entire South-American distribution. No evidence of isolation-by-distance could be detected for this species in South America.ConclusionsDespite their pronounced bimodal morphologies and different lifestyles, the gregaria s. str. and subrugosa ecotypes form a single, dimorphic species M. gregaria s. l.. Based on adequate geographic coverage and multiple independent polymorphic loci, there is no indication that each phenotype may have a unique genetic basis, leaving phenotypic plasticity or localized genomic islands of speciation as possible explanations.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-016-0836-4) contains supplementary material, which is available to authorized users.
Highlights
DNA barcoding has demonstrated that many discrete phenotypes are genetically distinctcryptic species
In this paper we investigate the dimorphic squat lobster, Munida gregaria sensu lato (Fabricius, 1793), which is currently considered to comprise the ecotypes M. gregaria sensu stricto Miers 1881 as well as its junior synonym M. subrugosa Dana, 1852
Based on extensive sampling of the species’ distribution in South America and using nine independent polymorphic nuclear microsatellite loci in addition to new mitochondrial c oxidase subunit I (COI) sequences, we were able to show that the lack of genetic differentiation between distinct gregaria s. str. and subrugosa ecotypes is not an artefact due to insufficient genomic and/or geographic sampling or slowly evolving markers
Summary
DNA barcoding has demonstrated that many discrete phenotypes are genetically distinct (pseudo)cryptic species. In order to show that too slowly evolving markers or other artefacts (e.g. mito-nuclear discordance [12]) did not trivially cause the observed lack of differentiation, considerably more extensive molecular evidence including multiple unlinked nuclear loci with sufficiently high substitution rates is required Such extensive a posteriori knowledge is rare (e.g. in the fully sequenced Daphnia pulex [13,14,15]), but numerous experimental studies in which the genetic identity of individuals is known a priori contribute greatly to our understanding of the importance of polyphenism and morphological plasticity, e.g. parthenogenetic aphids [16, 17], marbled crayfish [18], polyembryonic armadillos [19], inbred lines of Drosophila [20], cloned swine [21]. It is unclear if the small number of confirmed polyphenism resulting from similar or identical genetic backgrounds is a condition truly rare in nature or whether it reflects mostly a discovery and/or publication bias
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