Testing heterogeneous base composition as potential cause for conflicting phylogenetic signal between mitochondrial and nuclear DNA in the land snail genus Theba Risso 1826 (Gastropoda: Stylommatophora: Helicoidea)

The systematics and evolution of the Sri Lankan rainforest land snail Corilla: New insights from RADseq-based phylogenetics

The stylommatophoran land-snail genus Corilla is endemic to Sri Lanka and India's Western Ghats. On the basis of habitat distribution and shell morphology, the 10 extant Sri Lankan species fall into two distinct groups, lowland and montane. Here, we use phylogenetic analyses of restriction-site-associated DNA sequencing (RADseq) data and ancestral-state reconstructions of habitat association and shell morphology to clarify the systematics and evolution of Sri Lankan Corilla. Our dataset consists of 9 species of Corilla. Phylogenetic analyses were based on 88 assemblies (9,604-4,132,850 bp) generated by the RADseq assembler ipyrad, using four parameter combinations and different levels of missing data. Trees were inferred using a maximum likelihood (ML) approach. Ancestral states were reconstructed using maximum parsimony (MP) and ML approaches, with 1 binary state character analysed for habitat association (lowland vs montane) and 6 binary state characters analysed for shell morphology (shape, colour, lip width, length of upper palatal folds, orientation of upper palatal folds and collabral sculpture). Over a wide range of missing data (40-87 % missing individuals per locus) and assembly sizes (62,279-4,132,850 bp), nearly all trees conformed to one of two topologies (A and B), most relationships were strongly supported and total branch support approached the maximal value. Apart from the position of Corilla odontophora 'south', topologies A and B showed similar, well-resolved relationships at and above the species level. Our study agrees with the shell-based taxonomy of C. adamsi, C. beddomeae, C. carabinata, C. colletti and C. humberti (all maximally supported as monophyletic species). It shows that C. erronea and C. fryae constitute a single relatively widespread species (for which the valid name is C. erronea) and that the names C. gudei and C. odontophora each apply to at least two distinct, yet conchologically-cryptic species. The MP and ML ancestral-state reconstructions yielded broadly similar results and provide firm evidence that diversification in Sri Lankan Corilla has involved evolutionary convergence in the shell morphology of lowland lineages, with a pale shell and wide lip having evolved on at least two separate occasions (in C. carabinata and C. colletti) from montane ancestors having a dark, narrow-lipped shell.

Diversity of Land Snail Tribe Helicini (Gastropoda: Stylommatophora: Helicidae): Where Do We Stand after 20 Years of Sequencing Mitochondrial Markers?

Sequences of mitochondrial genes revolutionized the understanding of animal diversity and continue to be an important tool in biodiversity research. In the tribe Helicini, a prominent group of the western Palaearctic land snail fauna, mitochondrial data accumulating since the 2000s helped to newly delimit genera, inform species-level taxonomy and reconstruct past range dynamics. We combined the published data with own unpublished sequences and provide a detailed overview of what they revealed about the diversity of the group. The delimitation of Helix is revised by placing Helix godetiana back in the genus and new synonymies are suggested within the genera Codringtonia and Helix. The spatial distribution of intraspecific mitochondrial lineages of several species is shown for the first time. Comparisons between species reveal considerable variation in distribution patterns of intraspecific lineages, from broad postglacial distributions to regions with a fine-scale pattern of allopatric lineage replacement. To provide a baseline for further research and information for anyone re-using the data, we thoroughly discuss the gaps in the current dataset, focusing on both taxonomic and geographic coverage. Thanks to the wealth of data already amassed and the relative ease with which they can be obtained, mitochondrial sequences remain an important source of information on intraspecific diversity over large areas and taxa.

Chromosome Diversity and Evolution in Helicoide a (Gastropoda: Stylommatophora): A Synthesis from Original and Literature Data

Simple Summary

The superfamily Helicoidea is a large and diverse group of Eupulmonata. The superfamily has been the subject of several molecular and phylogenetic studies which greatly improved our knowledge on the evolutionary relationships and historical biogeography of many families. In contrast, the available karyological information on Helicoidea still results in an obscure general picture, lacking a homogeneous methodological approach and a consistent taxonomic record. Nevertheless, the available karyological information highlights the occurrence of a significant chromosomal diversity in the superfamily in terms of chromosome number (varying from 2n = 40 to 2n = 62), chromosome morphology and the distribution of different karyological features among different taxonomic groups. Here we performed a molecular and a comparative cytogenetic analysis on of 15 Helicoidea species of three different families. Furthermore, to provide an updated assessment of the chromosomal diversity of the superfamily we reviewed all the available chromosome data. Finally, superimposing all the chromosome data gathered from different sources on the available phylogenetic relationships of the studied taxa, we discuss the overall observed chromosome diversity in Helicoidea and advance a hypothesis on its chromosomal evolution.

Abstract

We performed a molecular and a comparative cytogenetic analysis on different Helicoidea species and a review of all the available chromosome data on the superfamily to provide an updated assessment of its karyological diversity. Standard karyotyping, banding techniques, and Fluorescence in situ hybridization of Nucleolus Organizer Region loci (NOR-FISH) were performed on fifteen species of three families: two Geomitridae, four Hygromiidae and nine Helicidae. The karyotypes of the studied species varied from 2n = 44 to 2n = 60, highlighting a high karyological diversity. NORs were on a single chromosome pair in Cernuella virgata and on multiple pairs in four Helicidae, representing ancestral and derived conditions, respectively. Heterochromatic C-bands were found on pericentromeric regions of few chromosomes, being Q- and 4′,6-diamidino-2-phenylindole (DAPI) negative. NOR-associated heterochromatin was C-banding and chromomycin A₃ (CMA₃) positive. Considering the available karyological evidence on Helicoidea and superimposing the chromosome data gathered from different sources on available phylogenetic inferences, we describe a karyotype of 2n = 60 with all biarmed elements as the ancestral state in the superfamily. From this condition, an accumulation of chromosome translocations led to karyotypes with a lower chromosome number (2n = 50–44). This process occurred independently in different lineages, while an augment of the chromosome number was detectable in Polygyridae. Chromosome inversions were also relevant chromosome rearrangements in Helicoidea, leading to the formation of telocentric elements in karyotypes with a relatively low chromosome count.

Multiple Introductions of the Pestiferous Land Snail Theba pisana (Müller, 1774) (Gastropoda: Helicidae) in Southern California

Simple Summary

In Southern California, USA, the introduced white Italian land snail, Theba pisana, is prolific and locally pestiferous. To better understand its diversity and infer its parent population(s), we collected it from Los Angeles and San Diego counties and generated and analyzed gene sequence data (CO1, 16S, ITS2) that we compared between localities and to T. pisana CO1 barcodes from around the world. We also compared the morphology of the jaw, radula, and reproductive systems in T. pisana from Los Angeles and San Diego Counties. We found that T. pisana living at several sites in Los Angeles County in 2019–2020 had a single origin and were most similar in CO1 DNA sequence, based on available data, to specimens from Malta. Theba pisana collected from one site in San Diego County differed from Los Angeles T. pisana and were most similar in CO1 barcode sequence to specimens from Morocco. Jaw and mucous gland morphology also differed between Los Angeles and San Diego populations, but it is unclear if these traits are unique to lineages of T. pisana or if they change during a snail’s lifetime. We discuss how Los Angeles and San Diego T. pisana lineages may have arrived in Southern California and anticipate that the genetic data and morphological observations generated by this study will inform future studies of T. pisana where it is native and introduced.

Abstract

The terrestrial land snail Theba pisana is circum-Mediterranean in native range and widely introduced and pestiferous in regions around the world. In California, USA, T. pisana has been recorded intermittently since 1914, but its source population(s) are unknown, and no morphological or molecular analyses within or between California populations have been published. Therefore, we compared molecular data (CO1, 16S, ITS2) and internal morphology (jaw, radula, reproductive system) in T. pisana collected from Los Angeles and San Diego counties in 2019–2020. DNA barcode (CO1 mtDNA) analysis revealed that T. pisana from Los Angeles County was most similar to T. pisana from the Mediterranean island of Malta, and northern San Diego County-collected specimens were most similar to T. pisana from Morocco. Morphology of the jaw and mucous glands also differed between Los Angeles and San Diego populations, but it is unclear if traits are lineage-specific or artifacts of ontogeny. Several pathways of introduction into Southern California are possible for this species, but evidence for intentional vs. accidental introduction of present populations is lacking. Subsequent investigation(s) could use the data generated herein to assess the provenance of T. pisana elsewhere in California and/or worldwide and inform analyses of reproductive biology and systematics in this widespread species.

Snails in the desert: Species diversification of Theba (Gastropoda: Helicidae) along the Atlantic coast of NW Africa

The spatial subdivision of species often plays a pivotal role in speciation. Across their entire range, species are rarely panmictic and crucial consequences of spatial subdivision are (1) random genetic drift including historical factors, (2) uniform selection, and (3) divergent selection. Each of these consequences may result in geographic variation and eventually reproductive isolation, but their relative importance in speciation is still unclear. In this study, we used a combination of genetic, morphological, and climatic data to obtain a comprehensive picture of differentiation among three closely related, parapatrically distributed taxa of the land snail genus Theba occurring along the Atlantic coasts of South Morocco and Western Sahara. We conducted Mantel and partial Mantel tests to relate phenotypic and genotypic variation of these species to geography and/or climate. As null hypothesis for an evolutionary scenario, we assumed nonadaptive speciation and expected a pattern of isolation by distance among taxa. Rejection of the null hypothesis would indicate isolation by environment due to adaptation. Generally, genetic drift plays an important role but is rarely considered as sole driver of speciation. It is the combination of drift and selection that predominantly drives speciation. This study, however, provides a potential example, in which nonadaptive speciation, that is, genetic drift, is apparently the main driver of shaping the diversity of Theba in NW Africa. Restriction of gene flow between populations caused by geographic isolation probably has played an important role. Climate oscillations during the Plio- and Pleistocene may have led to repeated ecological changes in NW Africa and disruptions of habitats promoting differentiation by geographic isolation. The inferred evolutionary scenario, however, did not fully explain the incongruence between the AFLP- and mtDNA-tree topologies. This incongruence might indicate past hybridization among the studied Theba forms.