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Researchers compared gene expression in reproductive cells of two closely related fruit fly species using single-nucleus sequencing of over 100,000 cells from testes and ovaries. They found that male germline cells showed highly uneven evolutionary divergence, with sperm-producing cells (spermatocytes) evolving rapidly while stem cells remained conserved, whereas female germline cells evolved more uniformly with broader conservation across cell types. Approximately 40% of genes showing species differences were expressed in only one cell type, and divergent cells contained younger genes evolving at faster rates, with ovaries showing patterns consistent with adaptive evolution on the X chromosome.
Why it matters
This work provides insights into how reproductive systems can evolve while maintaining fertility between closely related species, which is fundamental to understanding speciation. The identification of specific cell types that drive evolutionary change versus those that remain conserved offers a framework for studying reproductive biology and infertility across species.
Understand the Science
by Imtiyaz E. Hariyani, Sudeshna Das, Emma M. Le, Carmen Gamero-Castano, Tina Soroudi, Joshua Choi, Spring Momeni, Justin C. Kim, Rongying Lu, Vivek Swarup, José M. Ranz
Reproductive organs vary widely across species yet share conserved cell types that produce gametes, sustaining species’ perpetuation. However, tissue-level comparisons mask critical differences among cell types, obscuring where evolutionary divergence occurs even between closely related species. We quantified expression divergence at cell-type resolution between two sibling species, Drosophila melanogaster and D. simulans, while disentangling adaptive and nonadaptive evolutionary mechanisms. We built a comparative single-nucleus transcriptomic atlas of over 100,000 nuclei from testes and ovaries of both species. Our analysis revealed sharply heterogeneous divergence across testis cell types, contrasting with a broader conservation across ovary cell types. Notably, in both organs, ~40% of genes showing interspecific differences did so in only one cell type. In the testis, spermatogonia were largely conserved, whereas divergence peaked in primary spermatocytes with extensive rewiring of coexpression modules linked to microtubule and mitochondrial functions. In the ovary, expression was largely conserved, except in early germline and late follicle cells, which showed shifts in oogenesis and cell-cycle-related coexpression modules. Divergent cell types in both tissues were enriched for evolutionarily young genes with narrow expression breadth and faster protein evolution rates. Additionally, the ovary exhibited a faster-X effect consistent with adaptive evolution. These findings reveal a fundamental asymmetry in how male and female germlines evolve, with functional constraints relaxed in specific testis cell types but broadly maintained across the ovary. Our work provides an evolutionary framework explaining how core reproductive functions are safeguarded during species diversification while identifying germline cells that drive evolutionary change.