Biology subjects

Burch, C. L.

Publications and source records attributed to Burch, C. L..

5 recordsLinked to original sources

bioRxiv2026

Modified meiosis in the tardigrade Hypsibius exemplaris maintains heterozygosity across the genome

In asexual reproduction, meiosis must be bypassed or altered to maintain ploidy from mother to daughter without fertilization. Most of the ways meiosis can be modified to this end are expected to reduce heterozygosity within individuals; however, many asexual species are highly heterozygous. Asexual reproduction is especially common among species of microscopic, desiccation-tolerant animals such as rotifers, nematodes, and tardigrades, but the cellular and genetic mechanisms underlying asexual reproduction have not been definitively documented in any species of tardigrade. Here, we show that the asexual tardigrade Hypsibius exemplaris fails to complete the cell division of meiosis I, followed by a complete meiosis II-like division, and reproduction proceeds without detectable loss of heterozygosity. We used a combined cytological and genomic approach to characterize the mechanism of reproduction and pattern of allele inheritance in this species. Furthermore, we identified heterozygous variants in a subset of transcriptionally active genes consistent with loss of function in one allele, suggesting that maintained heterozygosity in this species allowed divergence between alleles over time. This work establishes the meiotic mechanism and inheritance pattern of reproduction in H. exemplaris, which provides a framework for interpreting genetic variation in this organism as a laboratory model. Additionally, our finding that meiosis is modified in H. exemplaris via a mechanism that maintains heterozygosity across the genome adds to a growing list of asexual animals that have evolved to reproduce clonally despite the expected long-term costs. Article SummaryAsexual animals must alter meiosis, a highly conserved process of two cell divisions normally used to make eggs and sperm. This study represents the first combined cytological and genetic characterization of how meiosis is modified in a tardigrade. The authors found that the model tardigrade Hypsibius exemplaris modifies meiosis by skipping cytokinesis of the first division, followed by a complete second division. They also found that this species preserves heterozygosity across the genome and from generation to generation. Finally, some genes show evidence of divergence between alleles, supporting a broader conclusion that maintaining heterozygosity influences how asexual species evolve.

genetics↗

bioRxiv2022

Genetic drift promotes and recombination hinders speciation on a holey fitness landscape

Dobzhansky and Muller proposed a general mechanism through which microevolution, the substitution of alleles within populations, can cause the evolution of reproductive isolation between populations and, therefore, macroevolution. As allopatric populations diverge, many combinations of alleles differing between them have not been tested by natural selection and may thus be incompatible. Such genetic incompatibilities often cause low fitness in hybrids between species. Furthermore, the number of incompatibilities grows with the genetic distance between diverging populations. However, what determines the rate and pattern of accumulation of incompatibilities remains unclear. We investigate this question by simulating evolution on holey fitness landscapes on which genetic incompatibilities can be identified unambiguously. We find that genetic incompatibilities accumulate more slowly among genetically robust populations and identify two determinants of the accumulation rate: recombination rate and population size. In large populations with abundant genetic variation, recombination selects for increased genetic robustness and, consequently, incompatibilities accumulate more slowly. In small populations, genetic drift interferes with this process and promotes the accumulation of genetic incompatibilities. Our results suggest a novel mechanism by which genetic drift promotes and recombination hinders speciation. Author summaryAs geographically isolated populations evolve, genetic incompatibilities accumulate between them. Eventually, these incompatibilities may cause the populations to become different species. What determines how quickly species are formed in this way remains unclear. We investigate this question using computer simulations and find that genetic incompatibilities accumulate more slowly among populations with individuals that are robust to genetic perturbations. We identify two factors that influence the accumulation rate via their effects on genetic robustness: the size of the populations and how much recombination takes place in them. Small populations with rare recombination accumulate incompatibilities more quickly.

evolutionary biology↗

bioRxiv2022

Genome-wide determination of barriers to horizontal gene transfer

Horizontal gene transfer (HGT) is a major contributor to bacterial genome evolution, generating phenotypic diversity, driving the expansion of protein families, and facilitating the evolution of new phenotypes, new metabolic pathways, and new species. Comparative studies of gene gain in bacteria suggest that the frequency with which individual genes successfully undergo HGT varies considerably and may be associated with the number of protein-protein interactions in which the gene participates--its connectivity. Two non-exclusive hypotheses have emerged to explain why transferability should decrease with connectivity: the Complexity Hypothesis (Jain, Rivera, & Lake, 1999) and the Balance Hypothesis (Papp, Pal, & Hurst, 2003). These hypotheses predict that the functional costs of HGT arise from a failure of divergent homologues to make normal protein-protein interactions or from gene mis-expression, respectively. Here we describe genome-wide assessments of these hypotheses in which we used 74 existing prokaryotic whole genome shotgun libraries to estimate rates of horizontal transfer of genes from taxonomically diverse prokaryotic donors into E. coli. We show that transferability declines as connectivity increases, but that this effect is driven primarily by the ribosomal genes. We also show that transferability declines as the divergence (% amino acid difference) between donor and recipient orthologs increases and that this effect of divergence increases with connectivity. We explain how these results, even the stronger effect of connectivity on the transferability of ribosomal compared to non-ribosomal genes, provide strong support for both the Balance and Complexity Hypotheses. Significance StatementComparisons between prokaryotic genomes consistently show that genes with informational functions, e.g. in genome replication, transcription, and translation, have been subject to horizontal gene transfer between species more often than genes with operational functions, e.g. in metabolism and environmental sensing. In this study, we use a genome wide analysis of transferability data obtained from 74 genomes to provide the first experimental evidence that this pattern results from differences between informational and operational genes in the number of other proteins with which they interact, i.e., their connectivity, rather than from their functional differences. The importance of our exceptionally large dataset to the detection of connectivity effects on transferability explains why past experimental studies failed to replicate the consistent finding from comparative genomic studies.

genomics↗

bioRxiv2021

When good mutations go bad: how population size can change the direction of natural selection

Classical evolutionary theory holds that the efficiency, but not the direction, of natural selection depends on population size. In small populations, drift overwhelms selection, rendering all fitness-affecting mutations selectively neutral. Yet, beneficial mutations never become deleterious and deleterious mutations never become beneficial. Remarkably, several mutations, including in modifiers of recombination and mutation rate, have now been shown to be favored at some population sizes but disfavored at others, challenging established theory. Previously, we have designated this phenomenon sign inversion. Here we show that, unlike selected mutations in the classical framework, mutations susceptible to sign inversion confer both fitness costs and fitness benefits, that vary among their carriers. Furthermore, all such mutations can be classified based on whether their effects differ between or within mutant lineages. Using computer simulations, we demonstrate that both between-lineage and within-lineage variability can cause sign inversion and elucidate the common underlying mechanism. Our results confirm that variability in the sign of selective effects is necessary for sign inversion, which occurs because drift overwhelms selection on carriers bearing the cost and carriers enjoying the benefit at different population sizes.

evolutionary biology↗

bioRxiv2018

Population structure promotes the evolution of costly sex in artificial gene networks

We build on previous observations that Hill-Robertson interference generates an advantage of sex that, in structured populations, can be large enough to explain the evolutionary maintenance of costly sex. We employed a gene network model that explicitly incorporates interactions between genes. Mutations in the gene networks have variable effects that depend on the genetic background in which they appear. Consequently, our simulations include two costs of sex--recombination and migration loads--that were missing from previous studies of the evolution of costly sex. Our results suggest a critical role for population structure that lies in its ability to align the long- and short-term advantages of sex. We show that the addition of population structure favored the evolution of sex by disproportionately decreasing the equilibrium mean fitness of asexual populations, primarily by increasing the strength of Mullers Ratchet. Population structure also increased the ability of the short-term advantage of sex to counter the primary limit to the evolution of sex in the gene network model--recombination load. On the other hand, highly structured populations experienced migration load in the form of Dobzhansky-Muller incompatibilities, decreasing the effective rate of migration between demes and, consequently, accelerating the accumulation of drift load in the sexual populations.

evolutionary biology↗