Karyology of the Antarctic chiton Nuttallochiton mirandus (Thiele, 1906) (Mollusca: Polyplacophora) with some considerations on chromosome evolution in chitons

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.

The Iron-Responsive Genome of the Chiton Acanthopleura granulata

Molluscs biomineralize structures that vary in composition, form, and function, prompting questions about the genetic mechanisms responsible for their production and the evolution of these mechanisms. Chitons (Mollusca, Polyplacophora) are a promising system for studies of biomineralization because they build a range of calcified structures including shell plates and spine- or scale-like sclerites. Chitons also harden the calcified teeth of their rasp-like radula with a coat of iron (as magnetite). Here we present the genome of the West Indian fuzzy chiton Acanthopleura granulata, the first from any aculiferan mollusc. The A. granulata genome contains homologs of many genes associated with biomineralization in conchiferan molluscs. We expected chitons to lack genes previously identified from pathways conchiferans use to make biominerals like calcite and nacre because chitons do not use these materials in their shells. Surprisingly, the A. granulata genome has homologs of many of these genes, suggesting that the ancestral mollusc may have had a more diverse biomineralization toolkit than expected. The A. granulata genome has features that may be specialized for iron biomineralization, including a higher proportion of genes regulated directly by iron than other molluscs. A. granulata also produces two isoforms of soma-like ferritin: one is regulated by iron and similar in sequence to the soma-like ferritins of other molluscs, and the other is constitutively translated and is not found in other molluscs. The A. granulata genome is a resource for future studies of molluscan evolution and biomineralization.

Novel repeated DNAs in the antarctic polyplacophoran Nuttallochiton mirandus (Thiele, 1906)

Within the scope of a project on the characterization of satellite DNAs in polar mollusks, the Antarctic chiton Nuttallochitonmirandus (Thiele, 1906) was analyzed. Two novel families of tandemly repeated DNAs, namely NmH and NmP, are described in their structure and chromosomal localization, and, furthermore, their presence was analyzed in related species. Data reported here display a particular variability in the structural organization of DNA satellites within this species. Processes driving satellite evolution, which are likely responsible for the intriguing variability of the identified satellite DNAs, are discussed.

Chromosomes of Lepidochitona caprearum (Scacchi, 1836) (Polyplacophora, Acanthochitonina, Tonicellidae) provide insights into Acanthochitonina karyological evolution

We describe the karyotype, location of nucleolus-organizing regions (NORs) and heterochromatin composition and distribution in Lepidochitona caprearum (Scacchi, 1836). The examined specimens had 2n=24 chromosomes; the elements of pairs 1-4 were metacentric, subtelocentric those of the fifth pair, telocentric the elements of other pairs. NOR-FISH, Ag-NOR- and CMA3 banding showed NORs localized on pericentromeric regions of a medium small sized, telocentric chromosome pair. After C-banding or digestions with restriction enzyme NOR associate heterochromatin only was cytologically evident, resulting CMA3 positive. The comparison with chromosome data of other chitons, other than to evidence a karyotypic similarity of Lepidochitona caprearum to species of suborder Acanthochitonina, allows us to infer that chromosome evolution in the suborder mainly occurred via reduction of the number of the chromosomes by centric fusions, which took place repeatedly and independently in the different lineages of Acanthochitonina.

Comparative analysis of chromosome counts infers three paleopolyploidies in the mollusca

The study of paleopolyploidies requires the comparison of multiple whole genome sequences. If the branches of a phylogeny on which a whole-genome duplication (WGD) occurred could be identified before genome sequencing, taxa could be selected that provided a better assessment of that genome duplication. Here, we describe a likelihood model in which the number of chromosomes in a genome evolves according to a Markov process with one rate of chromosome duplication and loss that is proportional to the number of chromosomes in the genome and another stochastic rate at which every chromosome in the genome could duplicate in a single event. We compare the maximum likelihoods of a model in which the genome duplication rate varies to one in which it is fixed at zero using the Akaike information criterion, to determine if a model with WGDs is a good fit for the data. Once it has been determined that the data does fit the WGD model, we infer the phylogenetic position of paleopolyploidies by calculating the posterior probability that a WGD occurred on each branch of the taxon tree. Here, we apply this model to a molluscan tree represented by 124 taxa and infer three putative WGD events. In the Gastropoda, we identify a single branch within the Hypsogastropoda and one of two branches at the base of the Stylommatophora. We also identify one or two branches near the base of the Cephalopoda.

Molecular and cytogenetic characterization of repetitive DNA in the Antarctic polyplacophoran Nuttallochiton mirandus

Two highly repeated DNAs, designated NmE1/NmE2 and NmE5, were identified by EcoRV digestion in the chiton Nuttallochiton mirandus (Mollusca: Polyplacophora). The comparison of the sequences obtained showed high similarity in 5' and 3' regions and the NmE5 sequence displayed an inserted sequence that might arise from a transposable element. Southern blotting analyses suggested a tandem organization of both satellite DNA families identified. Moreover, dot blot analyses, performed on several molluscan species, revealed a different degree of conservation of the repeated DNAs. Fluorescence in-situ hybridizations (FISH) on metaphase chromosomes showed that both satellite DNAs are located at centromeric regions.