Assessment of mitochondrial genomes for heterobranch gastropod phylogenetics

Abstract

Background Heterobranchia is a diverse clade of marine, freshwater, and terrestrial gastropod molluscs. It includes such disparate taxa as nudibranchs, sea hares, bubble snails, pulmonate land snails and slugs, and a number of (mostly small-bodied) poorly known snails and slugs collectively referred to as the “lower heterobranchs”. Evolutionary relationships within Heterobranchia have been challenging to resolve and the group has been subject to frequent and significant taxonomic revision. Mitochondrial (mt) genomes can be a useful molecular marker for phylogenetics but, to date, sequences have been available for only a relatively small subset of Heterobranchia. Results To assess the utility of mitochondrial genomes for resolving evolutionary relationships within this clade, eleven new mt genomes were sequenced including representatives of several groups of “lower heterobranchs”. Maximum likelihood analyses of concatenated matrices of the thirteen protein coding genes found weak support for most higher-level relationships even after several taxa with extremely high rates of evolution were excluded. Bayesian inference with the CAT + GTR model resulted in a reconstruction that is much more consistent with the current understanding of heterobranch phylogeny. Notably, this analysis recovered Valvatoidea and Orbitestelloidea in a polytomy with a clade including all other heterobranchs, highlighting these taxa as important to understanding early heterobranch evolution. Also, dramatic gene rearrangements were detected within and between multiple clades. However, a single gene order is conserved across the majority of heterobranch clades. Conclusions Analysis of mitochondrial genomes in a Bayesian framework with the site heterogeneous CAT + GTR model resulted in a topology largely consistent with the current understanding of heterobranch phylogeny. However, mitochondrial genomes appear to be too variable to serve as good phylogenetic markers for robustly resolving a number of deeper splits within this clade.

Scratching the Itch: Updated Perspectives on the Schistosomes Responsible for Swimmer’s Itch around the World

“…As has been discussed before [ 5 ], and is also accentuated in Figure 4 , as presently known, schistosomes collectively have exploited gastropods from 16 different families in the two most derived lineages of gastropods: the Caenogastropoda and the Heterobranchia [ 36 , 46 , 47 ]. Caenogastropods ranging from basal to derived representatives have been exploited by schistosomes, whereas amongst heterobranchs, most schistosomes have exploited the relatively derived freshwater Hygrophila lineage.…”

“…The most basal schistosomes are known from marine caenogastropod snails, but more derived schistosomes largely diversified in freshwater gastropods, including freshwater or amphibious caenogastropods, with most but not all species infecting heterobranchs, particularly freshwater pulmonates. Among the most derived avian schistosome lineages, evidence consistent with host switches from freshwater back to marine gastropods have occurred on at least two occasions [ 28 , 29 ], in each case involving switches into marine heterobranchs (Siphonariida and Cephalaspidea) that are by no means close relatives of the pulmonate heterobranchs [ 47 ].…”

Mitogenomes do not substantially improve phylogenetic resolution in a young non-model adaptive radiation of freshwater gastropods

“…In particular, there is a multitude of mitogenomes available, with many of them having been analysed to illuminate deeper evolutionary splits within particular gastropod lineages (e.g. [ 2 , 9 – 12 ]). At the interspecific level, however, traditional mitochondrial barcoding markers (COX1 and 16S rRNA) still dominate the field of phylogenetics and phylogeography, typically in combination with a few fast-evolving nuclear markers.…”

Molecular species delimitation refines the taxonomy of native and nonnative physinine snails in North America

“…Increasingly, molecular tools are being applied to resolve taxonomic and conservation issues among taxonomic divisions within gastropods 17 , but this has also been unevenly applied and contentious. Although the mitochondrial genome, often the workhorse for phylogenetic efforts because of its successful application to the majority of animal taxa 18 , has been problematic for revealing deep phylogenetic structure in gastropods 19 – 21 , it has been effective for detecting relationships among genera 22 and for assignment of individuals to species 10 , the latter the primary goal of DNA barcoding 23 . Mitochondrial sequences are increasingly used as a first approximation for delimiting species among taxonomically challenging groups 24 , but this application is rendered more difficult in analyses of gastropods because of uncertainty about what constitutes the transition from intraspecific variation to interspecific differences.…”

“…In part, this may have arisen because some physinine snails are ecological generalists whose appearance is plastic in response to environmental characteristics and the presence of predators 4 , 32 . All are capable of self-fertilization, which can contribute to rapid evolution and lead to the long branches associated with Physinae in several phylogenies 21 . Such divergence may be accentuated in isolated or thermally enhanced habitats where founder effects are pronounced, populations are small, and generation times may be short 33 .…”