Biology subjects

Coke, A. N.

Publications and source records attributed to Coke, A. N..

3 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↗

bioRxiv2025

Chromosomes remain individualized through interphase in embryos of the tardigrade Hypsibius exemplaris

Tardigrades are microscopic animals that can survive exceptional levels of ionizing radiation or desiccation - DNA-damaging conditions that would kill most animals. Irradiation or radiomimetic drug treatment of the tardigrade Hypsibius exemplaris can induce remarkably high expression levels of DNA repair genes, primarily those in the base excision repair and nonhomologous end joining pathways. How tardigrades can repair widespread DNA damage without producing frequent, large-scale chromosome structural abnormalities, like chromosome translocations and fusions, is unknown. Here, we report the results of examining chromosome and nuclear architecture throughout the cell cycle in early embryos of H. exemplaris. We found that H. exemplaris chromosomes are maintained in an individualized form throughout the cell cycle. We were surprised to also find that each chromosome is housed in a fully or partially separate lamin-lined compartment, instead of all chromosomes being housed in a single, nearly spherical nuclear lamina and envelope. Our results reveal unusual chromosomal and nuclear organization in a tardigrade. We speculate that these unexpected features might limit chromosomal rearrangements during DNA damage repair in extreme conditions. SIGNIFICANCE STATEMENTO_LIWe have investigated unusual chromosome organization in an organism that survives extremely DNA-damaging environments, a tardigrade, through early embryonic cell cycles. C_LIO_LIChromosomes in fixed and stained embryos appeared more condensed through interphase than is typical for animal cells. C_LIO_LIChromosomes remained individualized, in fully or partially separate lamin-lined compartments, through interphase. C_LIO_LIThe results reveal a unique nuclear and chromosomal organization in tardigrades, which we speculate might contribute to limiting chromosomal structure abnormalities under DNA damaging conditions in nature. C_LI

cell biology↗

bioRxiv2023

Transcription-directed membrane association organizes the chloroplast nucleoid structure

DNA is organized into chromatin-like structures, which support the maintenance and regulation of genomes. A unique and poorly understood form of DNA packaging exists in chloroplasts, which are endosymbiotic organelles responsible for photosynthesis. Chloroplast genomes, together with associated proteins, form membraneless structures known as nucleoids. The internal arrangement of the nucleoid, molecular mechanisms of DNA packaging, and connections between nucleoid structure and gene expression remain mostly unknown. We show that Arabidopsis thaliana chloroplast nucleoids have a unique organization driven by DNA binding to the thylakoid membranes. DNA associated with the membranes has high protein occupancy, reduced DNA accessibility, and is highly transcribed. In contrast, genes with low levels of transcription are further away from the membranes, have lower protein occupancy, and higher DNA accessibility. Disruption of transcription at specific genes in sigma factor mutants causes a corresponding reduction in membrane association, indicating that RNA polymerase activity causes DNA tethering to the membranes. We propose that transcription organizes the chloroplast nucleoid into a transcriptionally active membrane-associated core and a less active periphery.

plant biology↗