Adding New Pieces to the Puzzle of Karyotype Evolution in Harttia (Siluriformes, Loricariidae): Investigation of Amazonian Species

A remarkable morphological diversity and karyotype variability can be observed in the Neotropical armored catfish genus Harttia. These fishes offer a useful model to explore both the evolution of karyotypes and sex chromosomes, since many species possess male-heterogametic sex chromosome systems and a high rate of karyotype repatterning. Based on the karyotype organization, the chromosomal distribution of several repetitive DNA classes, and the rough estimates of genomic divergences at the intraspecific and interspecific levels via Comparative Genomic Hybridization, we identified shared diploid chromosome numbers (2n = 54) but different karyotype compositions in H. dissidens (20m + 26sm + 8a) and Harttia sp. 3 (16m + 18sm + 14st + 6a), and different 2n in H. guianensis (2n = 58; 20m + 26sm + 2st + 10a). All species further displayed similar patterns of chromosomal distribution concerning constitutive heterochromatin, 18S ribosomal DNA (rDNA) sites, and most of the surveyed microsatellite motifs. Furthermore, differences in the distribution of 5S rDNA sites and a subset of microsatellite sequences were identified. Heteromorphic sex chromosomes were lacking in H. dissidens and H. guianensis at the scale of our analysis. However, one single chromosome pair in Harttia sp. 3 males presented a remarkable accumulation of male genome-derived probe after CGH, pointing to a tentative region of early sex chromosome differentiation. Thus, our data support already previously outlined evidence that Harttia is a vital model for the investigation of teleost karyotype and sex chromosome dynamics.

Genome Assembly and Sex-Determining Region of Male and Female Populus × sibirica

The genus Populus is presented by dioecious species, and it became a promising object to study the genetics of sex in plants. In this work, genomes of male and female Populus × sibirica individuals were sequenced for the first time. To achieve high-quality genome assemblies, we used Oxford Nanopore Technologies and Illumina platforms. A protocol for the isolation of long and pure DNA from young poplar leaves was developed, which enabled us to obtain 31 Gb (N50 = 21 kb) for the male poplar and 23 Gb (N50 = 24 kb) for the female one using the MinION sequencer. Genome assembly was performed with different tools, and Canu provided the most complete and accurate assemblies with a length of 818 Mb (N50 = 1.5 Mb) for the male poplar and 816 Mb (N50 = 0.5 Mb) for the female one. After polishing with Racon and Medaka (Nanopore reads) and then with POLCA (Illumina reads), assembly completeness was 98.45% (87.48% duplicated) for the male and 98.20% (76.77% duplicated) for the female according to BUSCO (benchmarking universal single-copy orthologs). A high proportion of duplicated BUSCO and the increased genome size (about 300 Mb above the expected) pointed at the separation of haplotypes in a large part of male and female genomes of P. × sibirica. Due to this, we were able to identify two haplotypes of the sex-determining region (SDR) in both assemblies; and one of these four SDR haplotypes, in the male genome, contained partial repeats of the ARR17 gene (Y haplotype), while the rest three did not (X haplotypes). The analysis of the male P. × sibirica SDR suggested that the Y haplotype originated from P. nigra, while the X haplotype is close to P. trichocarpa and P. balsamifera species. Moreover, we revealed a Populus-specific repeat that could be involved in translocation of the ARR17 gene or its part to the SDR of P. × sibirica and other Populus species. The obtained results expand our knowledge on SDR features in the genus Populus and poplar phylogeny.

Sex-specific variation in the genome-wide recombination rate

In most species that reproduce sexually, successful gametogenesis requires recombination during meiosis. The number and placement of crossovers (COs) vary among individuals, with females and males often presenting the most striking contrasts. Despite the recognition that the sexes recombine at different rates (heterochiasmy), existing data fail to answer the question of whether patterns of genetic variation in recombination rate are similar in the two sexes. To fill this gap, we measured the genome-wide recombination rate in both sexes from a panel of wild-derived inbred strains from multiple subspecies of house mice (Mus musculus) and from a few additional species of Mus. To directly compare recombination rates in females and males from the same genetic backgrounds, we applied established methods based on immunolocalization of recombination proteins to inbred strains. Our results reveal discordant patterns of genetic variation in the two sexes. Whereas male genome-wide recombination rates vary substantially among strains, female recombination rates measured in the same strains are more static. The direction of heterochiasmy varies within two subspecies, Mus musculus molossinus and Mus musculus musculus. The direction of sex differences in the length of the synaptonemal complex and CO positions is consistent across strains and does not track sex differences in genome-wide recombination rate. In males, contrasts between strains with high recombination rate and strains with low recombination rate suggest more recombination is associated with stronger CO interference and more double-strand breaks. The sex-specific patterns of genetic variation we report underscore the importance of incorporating sex differences into recombination research.

The female (XX) and male (YY) genomes provide insights into the sex determination mechanism in spinach

Sexual reproduction is the primary means of reproduction for the vast majority of macroscopic organisms, including almost all animals and plants. Sex chromosomes are predicted to play a central role in sexual dimorphism. Sex determination in spinach is controlled by a pair of sex chromosomes. However, the mechanisms of sex determination in spinach remain poorly understand. Here, we assembled the genomes of both a female (XX) and a male (YY) individual of spinach, and the genome sizes were 978 Mb with 28,320 predicted genes and 926 Mb with 26,537 predicted genes, respectively. Based on reported sex-linked markers, chromosomes 4 of the female and male genome were defined as the X and Y chromosomes, and a 10 Mb male-specific region of the Y chromosome (MSY) from approximately 95– 105 Mb, was identified that contains abundant transposable elements (92.32%). Importantly, a large-scale inversion of about 13 Mb in length was detected on the X chromosome, corresponding to ~9 Mb and ~4 Mb on the Y chromosome, which were located on both sides of the MSY with two distinct evolutionary strata. Almost all sex-linked/Y-specific markers were enriched on the inversions/MSY, suggesting that the flanked inversions might result in recombination suppression between the X and Y chromosomes to maintain the MSY. Forty-nine genes within the MSY had functional homologs elsewhere in the autosomal region, suggesting movement of genes onto the MSY. The X and Y chromosomes of spinach provide a valuable resource for investigating spinach sex chromosomes evolution from wild to cultivated spinach and also provide a broader understanding of the sex determination model in the Amaranthaceae family.

Feminisation of complex traits in Drosophila melanogaster via female-limited X chromosome evolution

A handful of studies have investigated sexually antagonistic constraints on obtaining sex-specific fitness optima, though exclusively through male-genome-limited evolution experiments. In this paper, we established a female-limited X chromosome evolution experiment, where we used an X chromosome balancer to enforce the inheritance of the X chromosome through the matriline, thus removing exposure to male selective constraints. This approach eliminates the effects of sexually antagonistic selection on the X chromosome, permitting evolution towards a single sex-specific optimum. After multiple generations of selection, we found strong evidence that body size and development time had moved towards a female-specific optimum, whereas reproductive fitness and locomotion activity remained unchanged. The changes in body size and development time are consistent with previous results, and suggest that the X chromosome is enriched for sexually antagonistic genetic variation controlling these traits. The lack of change in reproductive fitness and locomotion activity could be due to a number of mutually non-exclusive explanations, including a lack of sexually antagonistic variance on the X chromosome or confounding effects of the use of the balancer chromosome. This study is the first to employ female-genome-limited selection and adds to the understanding of the complexity of sexually antagonistic genetic variation.

Different from tracheophytes, liverworts commonly have mixed 35S and 5S arrays

Background and Aims Unlike other nuclear genes in eukaryotes, rDNA genes (5S and 35S loci) are present in numerous copies per cell and, when stained, can therefore provide basic information about genome organization. In tracheophytes (vascular plants), they are usually located on separate chromosomes, the so-called S-type organization. An analysis of 1791 species of land plants suggested that S-type arrays might be ancestral in land plants, while linked (L-type) organization may be derived. However, no outgroup and only a handful of ferns and bryophytes were included. Methods We analysed genome sizes and the distribution of telomere, 5S and 35S rDNA FISH signals in up to 12 monoicous or dioicous species of liverworts from throughout a phylogeny that includes 287 of the 386 currently recognized genera. We also used the phylogeny to plot chromosome numbers and the occurrence of visibly distinct sex chromosomes. Key Results Chromosome numbers are newly reported for the monoicous Lejeunea cavifolia and for females of the dioicous Scapania aequiloba. We detected sex-related differences in the number of rDNA signals in the dioicous Plagiochila asplenioides and Frullania dilatata. In the latter, the presence of two UU chromosomes in females and additional 5S-35S rDNA loci result in a haploid genome 0.2082 pg larger than the male genome; sex-specific genome differences in the other dioicous species were small. Four species have S-type rDNA, while five species have mixed L-S rDNA organization, and transitions may have occurred multiple times, as suggested by rDNA loci not being conserved among closely related species of Pellia. All species shared an Arabidopsis-like telomere motif, and its detection allowed verification of the chromosome number of Radula complanata and chromosome rearrangements in Aneura pinguis and P. asplenioides, the latter also showing sex-specific interstitial telomere repeats. Conclusions The S and L rDNA arrangements appear to have evolved repeatedly within liverworts, even in the same species. Evidence for differential accumulation of rDNA between the sexes so far is limited.

Genome Survey of Male and Female Spotted Scat (Scatophagus argus)

The spotted scat, Scatophagus argus, is a species of fish that is widely propagated within the Chinese aquaculture industry and therefore has significant economic value. Despite this, studies of its genome are severely lacking. In the present study, a genomic survey of S. argus was conducted using next-generation sequencing (NGS). In total, 55.699 GB (female) and 51.047 GB (male) of high-quality sequence data were obtained. Genome sizes were estimated to be 598.73 (female) and 597.60 (male) Mbp. The sequence repeat ratios were calculated to be 27.06% (female) and 26.99% (male). Heterozygosity ratios were 0.37% for females and 0.38% for males. Reads were assembled into 444,961 (female) and 453,459 (male) contigs with N50 lengths of 5,747 and 5,745 bp for females and males, respectively. The average guanine-cytosine (GC) content of the female genome was 41.78%, and 41.82% for the male. A total of 42,869 (female) and 43,283 (male) genes were annotated to the non-redundant (NR) and SwissProt databases. The female and male genomes contained 66.6% and 67.8% BUSCO core genes, respectively. Dinucleotide repeats were the dominant form of simple sequence repeats (SSR) observed in females (68.69%) and males (68.56%). Additionally, gene fragments of Dmrt1 were only observed in the male genome. This is the first report of a genome-wide characterization of S. argus.

Chromosome level draft genomes of the fall armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae), an alien invasive pest in China

The fall armyworm (FAW), Spodoptera frugiperda (J.E. Smith) is a severely destructive pest native to the Americas, but has now become an alien invasive pest in China, and causes significant economic loss. Therefore, in order to make effective management strategies, it is highly essential to understand genomic architecture and its genetic background. In this study, we assembled two chromosome scale genomes of the fall armyworm, representing one male and one female individual procured from Yunnan province of China. The genome sizes were identified as 542.42 Mb with N50 of 14.16 Mb, and 530.77 Mb with N50 of 14.89 Mb for the male and female FAW, respectively. We predicted about 22,201 genes in the male genome. We found the expansion of cytochrome P450 and glutathione s-transferase gene families, which are functionally related to the intensified detoxification and pesticides tolerance. Further population analyses of corn strain (C strain) and rice strain (R strain) revealed that the Chinese fall armyworm was most likely invaded from Africa. These strain information, genome features and possible invasion source described in this study will be extremely important for making effective strategies to manage the fall armyworms.