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Meiotic I metaphases of males with the heterochromosome stained with chromomycin A3/methyl green. The heteromorphic pair appears precociously separated (a), as a cross-shaped tetrad (b), recombined in the chromosomal region between the two heterochromatic areas and terminally associated to form a ring tetrad (c).
Source publication
The karyotype of the isopod crustacean Asellus aquaticus does not normally display any heteromorphic sex chromosome pair. Some of the males in a wild population of A. aquaticus collected in the Sarno river near Naples do display a heteromorphic chromosome pair. The hetero-morphism is due to the presence of two intercalary heterochromatic areas on o...
Citations
... Another potential issue with the identification of genes with allele-specific expression stems from a lack of genomic information in A. aquaticus; we do not know whether the entire genome is diploid. One possible scenario in which part of the genome could be haploid is that A. aquaticus might have evolving sex chromosomes [39]. With the six genes we further genotyped to place on the map, we saw expected genotypes in the backcross, heterozygous or homozygous for the cave allele. ...
Background
Transcriptomic methods can be used to elucidate genes and pathways responsible for phenotypic differences between populations. Asellus aquaticus is a freshwater isopod crustacean with surface- and cave-dwelling ecomorphs that differ greatly in multiple phenotypes including pigmentation and eye size. Multiple genetic resources have been generated for this species, but the genes and pathways responsible for cave-specific characteristics have not yet been identified. Our goal was to generate transcriptomic resources in tandem with taking advantage of the species’ ability to interbreed and generate hybrid individuals.
Results
We generated transcriptomes of the Rakov Škocjan surface population and the Rak Channel of Planina Cave population that combined Illumina short-read assemblies and PacBio Iso-seq long-read sequences. We investigated differential expression at two different embryonic time points as well as allele-specific expression of F1 hybrids between cave and surface individuals. RNAseq of F2 hybrids, as well as genotyping of a backcross, allowed for positional information of multiple candidate genes from the differential expression and allele-specific analyses.
Conclusions
As expected, genes involved in phototransduction and ommochrome synthesis were under-expressed in the cave samples as compared to the surface samples. Allele-specific expression analysis of F1 hybrids identified genes with cave-biased (cave allele has higher mRNA levels than the surface allele) and surface-biased expression (surface allele has higher mRNA levels than the cave allele). RNAseq of F2 hybrids allowed for multiple genes to be placed to previously mapped genomic regions responsible for eye and pigmentation phenotypes. In the future, these transcriptomic resources will guide prioritization of candidates for functional analysis.
... On the one hand, the karyotype of A. aquaticus consists of eight homomorphic chromosome pairs (2n = 16) in both sexes (Montalenti and Rocchi, 1964;Salemaa, 1979;Valentino et al., 1983). However, the presence of a difference in heterochromatin areas for one chromosome pair in males from an Italian population has been taken as an indication for an incipient sex chromosome differentiation in A. aquaticus (Rocchi et al., 1984;Volpi et al., 1992). On the other hand, analyses of hybrid offspring between populations differing in male to female ratios indicate that sex ratio in A. aquaticus can be highly dependant on the origin of the mother (Vitagliano et al., 1994). ...
Interactions between organisms and their environments are central to how biological diversity arises and how natural populations and ecosystems respond to environmental change. These interactions involve processes by which phenotypes are affected by or respond to external conditions (e.g., via phenotypic plasticity or natural selection) as well as processes by which organisms reciprocally interact with the environment (e.g., via eco-evolutionary feedbacks). Organism-environment interactions can be highly dynamic and operate on different hierarchical levels, from genes and phenotypes to populations, communities, and ecosystems. Therefore, the study of organism-environment interactions requires integrative approaches and model systems that are suitable for studies across different hierarchical levels. Here, we introduce the freshwater isopod Asellus aquaticus, a keystone species and an emerging invertebrate model system, as a prime candidate to address fundamental questions in ecology and evolution, and the interfaces therein. We review relevant fields of research that have used A. aquaticus and draft a set of specific scientific questions that can be answered using this species. Specifically, we propose that studies on A. aquaticus can help understanding (i) the influence of host-microbiome interactions on organismal and ecosystem function, (ii) the relevance of biotic interactions in ecosystem processes, and (iii) how ecological conditions and evolutionary forces facilitate phenotypic diversification.
... The heteromorphism is due to the presence, on one chromosome of the pair, of two intercalary heterochromatic areas, one on each arm, which stain brightly with chromomycin A 3 (CMA). These two areas, as well as an interindividual variable number of CMA-positive telomeric heterochromatic regions, harbour the rRNA genes (Volpi et al. 1992, Barzotti et al. 1996). Moreover, an accumulation of the (TCC) n simple repeat sequence is present on the short arm of this chromosome in the region between the two heterochromatic areas (Volpi Chromosome Research 8: 459^464, 2000. ...
... The heteromorphic pair is present only in a proportion of the males of a natural population and thus seems to constitute a case of early stage of sex chromosome differentiation. Two heterochromatic areas harbouring ribosomal genes, and a signi¢cant accumulation of the repetitive simple sequence (TCC) n are present on the differentiated Y chromosome (Volpi et al. 1992(Volpi et al. , 1995. ...
... Non-sex-speci¢c highly repetitive sequences are evidently harboured in the heterochromatic telomeric regions and in the heterochromatic intercalary regions of the heteromorphic Y chromosome that contain rDNA and that are intensely labelled by both male-and female-derived probes also with a very large quantity of competitor DNA (Volpi et al. 1992, Barzotti et al. 1996. ...
In this work, genomic in-situ hybridization (GISH) was used to study the sex chromosome molecular differentiation on chromosomes of male and female individuals of the isopod crustacean Asellus aquaticus. As a composite hybridization probe, we contemporaneously used male and female whole genomic DNA differently labelled in the presence of an excess of unlabelled DNA of the female homogametic sex. The karyotype of A. aquaticius normally displays eight homomorphic chromosome pairs, but a heteromorphic sex chromosome pair is present in about a quarter of the males of a natural population previously identified by us. GISH did not reveal any sex chromosome molecular differentiation on the male and female homomorphic sex chromosome pair, and the karyotypes of these individuals were equally labelled by the male- and female-derived probe, while the heteromorphic Y chromosome showed a differentially labelled region only with the male-derived probe. This region evidently contains male-specific sequences but, because no similar hybridized region is observed on the male homomorphic chromosome pair, they are probably not important for sex determination but represent a molecular differentiation acquired from the Y chromosome.
... This chromosome is inherited through the male line as a normal Y chromosome and probably represents an early stage of morphological differentiation of a sex chromosome. Volpi et al.(1992) demonstrated that variant ribosomal genes are associated with its interstitial heterochromatic regions. ...
... These two types of areas have apparently the same organization. This fact confirms our theory that they have a common origin (Volpi et al. 1992). ...
... This is also supported by the very rare presence of a nucleolus associated with them. The low level of functioning of these genes could be linked to the demonstrable existence among them of gene variants (Volpi et al. 1992), or else to their unusual chromosomal location. ...
In the present investigation chromosomal preparations of Asellus aquaticus were sequentially stained with chromomycin A3 to reveal the heterochromatic areas, hybridized in situ with rDNA probes in order to map the ribosomal genes and finally silver stained to check the transcriptional activity of these genes. The results indicate the existence of a substantial correspondence of location and size among the heterochromatic regions and the regions over which the in situ hybridization signals spread. The ribosomal genes, quite independently of their location in the secondary constriction, can be silver stained and thus appear to be transcriptionally active. The ribosomal sequences also hybridize to the entire heterochromatic areas observed on the probable Y chromosome identified in some males of a natural population. These rRNA genes are only rarely transcriptionally active.
... A heteromorphic chromosome pair was identified in several males from a population collected from the Sarno river near Naples. One chromosome shows two intercalary heterochromatic areas, one on each arm, which stain brightly with chromomycin A3; variant ribosomal sequences are associated with these areas (Volpi et a!., 1992). This heterochromosome is inherited through the male line as a normal Y chromosome (Rocchi eta!., 1984). ...
... Exchange also occurs very infrequently in the chromosome area situated between the two heterochromatic areas, thus giving rise to chromosomes displaying only one heterochromatic area. On the strength of these and other observations (Volpi et a!., 1992), we may reasonably postulate an early stage of chromosome differentiation; that this differentiation does not involve chromosome rearrangements in the form of inversions but, probably, the transfer of heterochromatic and ribosomal sequences from telomeric regions to interstitial sites; and that this transfer could be favoured by the 'Rabi-orientation' that the chromosomes retain during interphase (Schweizer & Loidl, 1987). In recent years the existence of sex-specific distribution patterns for various simple, tandemly repeated DNA sequences has been demonstrated in a number of organisms. ...
... In previous papers we observed that certain males in an A. aquaticus population collected from the Sarno river near Naples display a heteromorphic sex chromosome pair. The Y chromosome has two intercalary areas that harbour ribosomal variant sequences (Rocchi et a!., 1984;Volpi et al., 1992). Meiotic recombination within the heteromorphic pair often occurs in the subterminal regions distal to the heterochromatic bands, but is very rarely observed in the region situated between them. ...
The crustacean isopod species Asellus aquaticus does not usually have recognizable sex chromosomes. We previously identified a Y chromosome marked by two heterochromatic bands in some males of a population from the Sarno river near Naples. In this work we used oligonucleotide probes to test the presence and possible sex-specific distribution of five simple repeat motifs in the genome of male and female individuals from the Sarno population. The five oligonucleotide probes were hybridized to enzyme-restricted genomic DNAs and the chromosome location of two probes was tested using fluorescence in situ hybridization. Our results show that only the (TCC)n repetitive simple sequence has a sex-specific hybridization pattern and presents a significant accumulation on the Y chromosome in the region included between the two heterochromatic areas. Moreover the GGAAT sequence is not present in the genome of A. aquaticus in any detectable quantity.Keywords: Asellus, crustacean, isopod, sex chromosome, simple repeats
... The heterochromosome has two intercalary hetero-*Correspondence 78 chromatic areas containing ribosomal sequences and is inherited as a normal Y chromosome. The ribosomal genes are usually telomeric and associated with heterochromatin (Rocchi et al., 1984;Volpi et at., 1992). ...
The repeated sequence TTAGGG is present at all tested vertebrate telomeres including those of humans and at the telomeres of evolutionarily very distant organisms such as trypanosomes and slime moulds. We tested for the presence of this sequence in the genome of the crustacean isopod Asellus aquaticus. Fluorescence in situ hybridization and BAL31 nuclease digestion demonstrate that the (TTAGGG)n sequence occurs at the extreme termini of the chromosomes and also at an interstitial site.Keywords: Asellus, BAL31 nuclease, crustacean, in situ hybridization, isopod, telomere
When a pair produce mainly or exclusively males this phenomenon is called arrhenogeny, thelygeny in the opposite case. Amphogeny defines the paritary sex ratio of the offspring (Vandel, 1941). The embryos of two geographical populations of Asellus aquaticus L. (Crust. Isop.) one French and one Italian and the hybrids of these two populations were bred and reared in standard conditions of temperature, nutrition and crowding at natural photoperiod. Female rate of the offspring is significantly different between French (50% ♀♀) and Italian (39% ♀♀). The majority of the French Asellus families were amphogenic, very few thelygenic and arrhenogenic. In the Italian Asellus families the arrhenogenic were very many and the thelygenic very few. In the hybrid offspring the female rate depend on the mother origin. These results led to suppose a «maternal inheritance» in sex determination: we suggest the cooperation between a cytoplasmic factor and a set of genes responsible of the sex determination. We suggest that the cytoplasmic factor (a mithocondrial DNA?) can inactivate randomly only one of the two sets of genes responsible of a sex.
Asellus aquaticus is an isopod crustacean whose chromosomes cannot be differentiated by G- or R-banding techniques. In this work, we have obtained a longitudinal differentiation of these chromosomes by in situ nick translation using restriction enzymes (HaeIII, DraI and BamHI) and DNase I digestions. The four nucleases, with different efficiencies, have produced similar labelling patterns. Staining with DAPI, Giemsa and chromomycin A3 reveals that the DNA of the nick-translated regions is generally more resistant to extraction from the chromosome. The results obtained on the heteromorphic sex chromosome pair observed in about a quarter of the males of a natural population allow several hypotheses to be advanced on the nature and origin of chromosome dimorphism.
