The complete mitochondrial genome sequence of Astyanax paranae(Teleostei: characiformes)

Mitochondrial genomes of five Hyphessobrycon tetras and their phylogenetic implications

To date, the taxonomic status and phylogenetic affinities within Hyphessobrycon, even among other genera in Characidae, remain unclear. Here, we determined five new mitochondrial genomes (mitogenomes) of Hyphessobrycon species (H. elachys, H. flammeus, H. pulchripinnis, H. roseus, and H. sweglesi). The mitogenomes were all classical circular structures, with lengths ranging from 16,008 to 17,224 bp. The type of constitutive genes and direction of the coding strand that appeared in the mitogenomes were identical to those of other species in Characidae. The highest value of the Ka/Ks ratio within 13 protein-coding genes (PCGs) was found in ND2 with 0.83, suggesting that they were subject to purifying selection in the Hyphessobrycon genus. Comparison of the control region sequences among seven Hyphessobrycon fish revealed that repeat units differ in length and copy number across different species, which led to sharp differences in mitogenome sizes. Phylogenetic trees based on the 13 PCGs did not support taxonomic relationships, as the Hyphessobrycon fish mixed with those from other genera. These data were combined to explore higher level relationships within Characidae and could aid in the understanding of the evolution of this group.

Comparative mitogenomes of three species in Moenkhausia: Rare irregular gene rearrangement within Characidae

Generally, a teleostean group possesses only one type or a set of similar mitochondrial gene arrangements. However, a new type of gene arrangement has been identified in the mitochondrial genomes (mitogenomes) of Moenkhausia. Here, three newly sequenced complete mitogenomes of tetras (Characidae: Moenkhausia) are presented (M. costae, M. pittieri, and M. sanctaefilomenae). The three mitogenomes had a classical circular structure, with total lengths ranging from 15,811 to 18,435 bp. Base composition analysis indicated that the sequences were biased toward adenine (A) and thymine (T), with A + T content of 54.63% in M. costae, 58.47% in M. pittieri, and 59.98% in M. sanctaefilomenae. The gene order and organization of M. sanctaefilomenae differed from those of typical teleostean mitogenomes. The genes tRNA-Ile, tRNA-Gln, and tRNA-Pro were translocated between tRNA-Trp and tRNA-Asn. One extra tRNA-Met and an extra CR were also discovered in the mitogenome. BI and ML analyses based on sequences of 38 different mitogenomes showed that M. costae and M. pittieri were classified together, and M. sanctaefilomenae was slightly further from other fish of the same genus. These results provide insight into the gene arrangement features of Characidae mitogenomes and lay the foundation for further phylogenetic studies on Characidae.

Long-term persistence of supernumerary B chromosomes in multiple species of Astyanax fish

BACKGROUND

Eukaryote genomes frequently harbor supernumerary B chromosomes in addition to the "standard" A chromosome set. B chromosomes are thought to arise as byproducts of genome rearrangements and have mostly been considered intraspecific oddities. However, their evolutionary transcendence beyond species level has remained untested.

RESULTS

Here we reveal that the large metacentric B chromosomes reported in several fish species of the genus Astyanax arose in a common ancestor at least 4 million years ago. We generated transcriptomes of A. scabripinnis and A. paranae 0B and 1B individuals and used these assemblies as a reference for mapping all gDNA and RNA libraries to quantify coverage differences between B-lacking and B-carrying genomes. We show that the B chromosomes of A. scabripinnis and A. paranae share 19 protein-coding genes, of which 14 and 11 were also present in the B chromosomes of A. bockmanni and A. fasciatus, respectively. Our search for B-specific single-nucleotide polymorphisms (SNPs) identified the presence of B-derived transcripts in B-carrying ovaries, 80% of which belonged to nobox, a gene involved in oogenesis regulation. Importantly, the B chromosome nobox paralog is expressed > 30× more than the A chromosome paralog. This indicates that the normal regulation of this gene is altered in B-carrying females, which could potentially facilitate B inheritance at higher rates than Mendelian law prediction.

CONCLUSIONS

Taken together, our results demonstrate the long-term survival of B chromosomes despite their lack of regular pairing and segregation during meiosis and that they can endure episodes of population divergence leading to species formation.

Differential Expression of Genes Related to Sexual Determination Can Modify the Reproductive Cycle of Astyanax scabripinnis (Characiformes: Characidae) in B Chromosome Carrier Individuals

The species complex Astyanax scabripinnis is one of the most studied with respect to origin, distribution, and frequency of B chromosomes, and is considered a model organism for evolutionary studies. Research using population inferences about the occurrence and frequency of the B chromosome shows seasonal variation between sexes, which is associated with the presence of this supernumerary element. We hypothesized that the B chromosome could influence the sex ratio of these animals. Based on this assumption, the present work aimed to investigate if differences exist among levels of gene expression with qRT-PCR of the amh (associated with testicular differentiation) and foxl2a (associated with ovarian differentiation) genes between B-carrier and non-B-carrier individuals. The results showed that for the amh gene, the difference in expression between animals with B chromosomes was not accentuated compared to that in animals without this chromosome. Expression of foxl2a in B-carrier females, however, was reduced by 73.56% compared to females that lacked the B chromosome. Males had no difference in expression of the amh and foxl2a genes between carriers and non-carriers of the B chromosome. Results indicate that the presence of B chromosomes is correlated with the differential expression of sex-associated genes. An analysis of these results integrated with data from other studies on the reproductive cycle in the same species reveals that this difference in expression may be expanding the reproductive cycle of the species.