Biology subjects

Sanchez-Ramirez, S.

Publications and source records attributed to Sanchez-Ramirez, S..

4 recordsLinked to original sources

bioRxiv2025

Phylogenomic timetree-calibrated speciation clocks for Caenorhabditis nematodes reveal slow but disproportionate accumulation of post-zygotic reproductive isolation

Reproductive isolation and genomic divergence both accumulate over time in the formation and persistence of distinct biological species. The pace of "speciation clocks" quantified with pre-zygotic and post-zygotic reproductive isolation, however, differs among taxa, with pre-zygotic isolation tending to evolve sooner in some but not all taxa. To address this issue in nematodes for the first time, here we infer the species tree and divergence times across the phylogeny of 51 species of Caenorhabditis. We incorporate several molecular evolutionary strategies in phylogenomic dating to account for complications in this group due to lack of fossil calibration, deep molecular divergence with synonymous-site saturation, and codon usage bias. By integrating divergence times with experimental data on pre- and post-zygotic reproductive isolation, we infer that post-zygotic isolation accumulates faster than pre-zygotic isolation in Caenorhabditis and that hybrid sterility evolves sooner than hybrid inviability. These findings are consistent with speciation being driven principally by intrinsic isolating barriers and the disproportionate fragility of germline developmental programs to disruption. We estimate that it takes approximately 50 million generations for intrinsic post-zygotic reproductive compatibility to be reduced by half, on average, between diverging pairs of Caenorhabditis. The protracted reproductive isolation clocks in Caenorhabditis may, in part, reflect the capacity to retain population genetic hyperdiversity, the incomplete sampling of global biodiversity, and as-yet uncharacterized incipient or cryptic species.

evolutionary biology↗

bioRxiv2021

COMPMAP: an allele-specific expression read-counter based on competitive mapping

SummaryChanges to regulatory sequences account for important phenotypic differences between species and populations. In heterozygote individuals, regulatory polymorphism typically manifests as allele-specific expression (ASE) of transcripts. ASE data from inter-species and inter-population hybrids, in conjunction with expression data from the parents, can be used to infer regulatory changes in cis and trans throughout the genome. Improper data handling, however, can create problems of mapping bias and excessive loss of information, which are prone to arise unintentionally from the cumbersome pipelines with multiple dependencies that are common among current methods. Here, we introduce a new, selfcontained method implemented in Python that generates allele-specific expression counts from genotype-specific map alignments. Rather than assessing individual SNPs, our approach sorts and counts reads within a given homologous region by comparing individual read-mapping statistics from each parental alignment. Reads that are aligned ambiguously to both references are resolved proportionally to the allele-specific matching read counts or statistically using a binomial distribution. Using simulations, we show CompMap has low error rates in assessing regulatory divergence. AvailabilityThe Python code with examples and installation instructions is available on the GitHub repository https://github.com/santiagosnchez/CompMap Contactsantiago.snchez@gmail.com Supplementary information

bioinformatics↗

bioRxiv2020

Sex-specific and sex-chromosome regulatory evolution underlie widespread misregulation of inter-species hybrid transcriptomes

When gene regulatory networks diverge between species, their dysfunctional expression in inter-species hybrid individuals can create genetic incompatibilities that generate the developmental defects responsible for intrinsic post-zygotic reproductive isolation. Both cis- and trans-acting regulatory divergence can be hastened by directional selection through adaptation, sexual selection, and inter-sexual conflict, in addition to cryptic evolution under stabilizing selection. Dysfunctional sex-biased gene expression, in particular, may provide an important source of sexually-dimorphic genetic incompatibilities. Here, we characterize and compare male and female/hermaphrodite transcriptome profiles for sibling nematode species Caenorhabditis briggsae and C. nigoni, along with allele-specific expression in their F1 hybrids, to deconvolve features of expression divergence and regulatory dysfunction. Despite evidence of widespread stabilizing selection on gene expression, misexpression of sex-biased genes pervades F1 hybrids of both sexes. This finding implicates greater fragility of male genetic networks to produce dysfunctional organismal phenotypes. Spermatogenesis genes are especially prone to high divergence in both expression and coding sequences, consistent with a "faster male" model for Haldanes rule and elevated sterility of hybrid males. Moreover, underdominant expression pervades male-biased genes compared to female-biased and sex-neutral genes and an excess of cis-trans compensatory regulatory divergence for X-linked genes underscores a "large-X effect" for hybrid male expression dysfunction. Extensive regulatory divergence in sex determination pathway genes likely contributes to demasculinization of XX hybrids. The evolution of genetic incompatibilities due to regulatory versus coding sequence divergence, however, are expected to arise in an uncorrelated fashion. This study identifies important differences between the sexes in how regulatory networks diverge to contribute to sex-biases in how genetic incompatibilities manifest during the speciation process. Authors summaryAs species diverge, many mutations that affect traits do so by altering gene expression. Such gene regulatory changes also accumulate in the control of static traits, due to compensatory effects of mutation on multiple regulatory elements. Theory predicts many of these changes to cause inter-species hybrids to experience dysfunctional gene expression that leads to reduced fitness, disproportionately affecting genes biased toward expression in one sex and that localize to sex chromosomes. Our analyses of genome-wide gene expression from Caenorhabditis nematode roundworms support these predictions. We find widespread rewiring of gene regulation, despite the extensive morphological stasis and conserved expression profiles that are hallmarks of these animals. Misregulation of expression in inter-species hybrids of both sexes is most severe for genes linked to the X-chromosome, but male organismal phenotypes are most disrupted in hybrids. This fragility of male genetic networks and sex differences in regulatory evolution of local versus distant elements may underlie feminized and sterile phenotypes among hybrids. Our work clarifies how distinct components of regulatory networks evolve and contribute to sex differences in the manifestation of genetic incompatibilities in the speciation process.

genomics↗

bioRxiv2017

Functional Bias and Demographic History Obscure Patterns of Selection among Single-Copy Genes in a Fungal Species Complex

Many different evolutionary processes may be responsible for explaining natural variation within genomes, some of which include natural selection at the molecular level and changes in population size. Fungi are highly adaptable organisms, and their relatively small genomes and short generation times make them pliable for evolutionary genomic studies. However, adaptation in wild populations has been relatively less documented compared to experimental or clinical studies. Here, we analyzed DNA sequences from 502 putative single-copy orthologous genes in 63 samples that represent seven recently diverged North American Amanita (jacksonii-complex) lineages. For each gene and each species, we measured the genealogical sorting index (gsi) and infinite-site-based summary statistics, such as [Formula], [Formula] and DTaj in coding and intron regions. MKT-based approaches and likelihood-ratio-test Kn/Ks models were used to measure natural selection in all coding sequences. Multi-locus (Extended) Bayesian Skyline Plots (eBSP) were used to model intraspecific demographic changes through time based on unlinked, putative neutral regions (introns). Most genes show evidence of long-term purifying selection, likely reflecting a functional bias implicit in single-copy genes. We find that two species have strongly negatively skewed Tajimas D, while three other have a positive skew, corresponding well with patterns of demographic expansion and contraction. Standard MKT analyses resulted in a high incidence of near-zero with a tendency towards negative values. In contrast, estimates based on the distribution of fitness effects (DFE), which accounts for demographic effects and slightly deleterious mutations, suggest a higher proportion of sites fixed by positive selection. The difference was more notorious in species with expansion signatures or with historically low population sizes, evidencing the concealing effects of specific demographic histories. Finally, we attempt to mitigate Gene Ontology term overrepresentation, highlighting the potential adaptive or ecological roles of some genes under positive selection.

evolutionary biology↗