Research

Research Overview

​Major research themes in our lab include evolutionary genetics, molecular evolution, sex chromosomes, reproductive biology, and sexual selection.  Particularly appealing are projects where these areas intersect.  Methodologically, we concentrate on functional, comparative, and population genomic approaches.  However, our work with these genome-wide data-sets is motivated by fundamental observations concerning organismal biology, such as variation in mating rates, sex differences in aging, and spermatogenesis. Taxonomically, we focus on insects, with a strong emphasis on moths and butterflies. Details regarding some of our recent and ongoing projects follow below.

Sex Chromosome Dosage Compensation

In species with differentiated sex chromosomes (e.g. with XY males or ZW females), the heterogametic sex has half the gene dose relative to the homogametic sex.  Theory predicts that epigenetic mechanisms should evolve to equilibrate gene expression on the X or Z chromosome relative to autosomes, and also between sexes. Yet empirical results show a variety of patterns exist in various taxa, often inconsistent with theory. Our lab has examined this prediction in Lepidoptera, showing that the Z chromosome is reduced relative to autosomes but balanced between sexes, a result inconsistent with theory. Furthermore, the pattern may differ between distinct segments of the Z chromosome, depending on ancestry. We are currently working to further assess this pattern in more species and also to uncover the molecular mechanisms underlying this pattern. At the same time, we are developing novel methods to improve and standardize relevant analytical approaches.

Representative Publications

1.Gu, L. & Walters, J. R. Evolution of Sex Chromosome Dosage Compensation in Animals: A Beautiful Theory, Undermined by Facts and Bedeviled by Details. Genome Biol Evol9, 2461–2476 (2017).

2.Gu, L. et al. Dichotomy of Dosage Compensation along the Neo Z Chromosome of the Monarch Butterfly. Curr Biol29, 4071-4077.e3 (2019).

3.Walters, J. R. & Hardcastle, T. J. Getting a full dose? Reconsidering sex chromosome dosage compensation in the silkworm, Bombyx mori. Genome Biol Evol3, 491–504 (2011).

Sex Chromosome Evolution

Beyond a focus on dosage compensation, we also are strongly interested in assessing other predictions concerning sex chromosome evolution. This includes faster-X, the role of sexual antagonism and sex-bias on gene content, as well as the origins and evolution of neo-sex chromosomes. Further characterizing the structure and diversity of W chromosomes is another focus in this area of our efforts.

Representative Publications

1.Mora, P. et al. Sex‐biased gene content is associated with sex chromosome turnover in Danaini butterflies. Mol. Ecol. e17256 (2024) doi:10.1111/mec.17256.

2.Mongue, A. J., Hansen, M. E. & Walters, J. R. Support for faster and more adaptive Z chromosome evolution in two divergent lepidopteran lineages. Evolution76, 332–345 (2022).

3.Mongue, A. J. & Walters, J. R. The Z chromosome is enriched for sperm proteins in two divergent species of Lepidoptera. Genome61, 248–253 (2018).

4.Wan, F. et al. A chromosome-level genome assembly of Cydia pomonella provides insights into chemical ecology and insecticide resistance. Nature Communications10, 1–14 (2019).

Mating system, sexual selection, and the molecular evolution of reproductive proteins

Reproductive proteins tend to diverge unusually rapidly between species. This pattern is frequently attributed to post-mating sexual selection.  Research in our lab examines the relationship between rates of molecular evolution and mating system using Lepidoptera.  Many lepidopteran species have good estimates of female mating rates based on counts of spermatophores from wild-caught individuals. Building on our proteomic identification of sperm and seminal fluid proteins, we use comparative and population genomic methods to infer pattern and process in the molecular evolution of reproductive proteins.

Representative Publications

1.Mongue, A. J., Hansen, M. E., Gu, L., Sorenson, C. E. & Walters, J. R. Nonfertilizing sperm in Lepidoptera show little evidence for recurrent positive selection. Molecular Ecology28, 2517–2530 (2019).

2.Walters, J. R., Stafford, C., Hardcastle, T. J. & Jiggins, C. D. Evaluating female remating rates in light of spermatophore degradation in Heliconius butterflies: pupal‐mating monandry versus adult‐mating polyandry. Ecological Entomology37, 257–268 (2012).

3.Walters, J. R. & Harrison, R. G. Decoupling of rapid and adaptive evolution among seminal fluid proteins in Heliconius butterflies with divergent mating systems. Evolution 65, 2855–2871 (2011).

Dimorphic sperm in Lepidoptera

Why would males make sperm – lots of them – that have no nucleus and can’t fertilize an egg?  This is the question motivating our effort to characterize the lepidopteran sperm proteome. Nearly all species of moths and butterflies produce two types of sperm. Eupyrene sperm are ‘normal’ in that they have nuclei and fertilize eggs.  In contrast, apyrene sperm lack nuclei, do not carry a haploid complement of the genome, and cannot directly fertilize eggs. We employ proteomics to characterize the molecular differences between apyrene and eupyrene sperm, combined with comparative and population genomic methods to understand the evolution of these proteins.

Representative Publications

1.Karr, T. L. & Walters, J. R. Panning for sperm gold: Isolation and purification of apyrene and eupyrene sperm from lepidopterans. Insect biochemistry and molecular biology63, 152–158 (2015).

2.Whittington, E., Karr, T. L., Mongue, A. J., Dorus, S. & Walters, J. R. Evolutionary Proteomics Reveals Distinct Patterns of Complexity and Divergence between Lepidopteran Sperm Morphs. Genome Biology and Evolution11, 1838–1846 (2019).

3.Yang, D. et al. BmPMFBP1 regulates the development of eupyrene sperm in the silkworm, Bombyx mori. Plos Genet18, e1010131 (2022).

Sex differences in aging

In nearly all animal species, sexes differ in their ageing and longevity. Often it is males who die sooner, but the pattern can vary greatly across species and what factors underly these sex-differences in aging is poorly understood. Our lab is researching the molecular basis of sex-differences in aging in Lepidoptera. As part of the IISAGE collaboration, we are collecting functional genomic and physiological data for several species to assess common features of sexual dimorphism in ageing and longevity.

Representative Publications

1.Riddle, N. C. et al. Comparative analysis of animal lifespan. GeroScience 1–11 (2023) doi:10.1007/s11357-023-00984-2.

2.Bronikowski, A. M. et al. Sex‐specific aging in animals: Perspective and future directions. Aging Cell21, e13542 (2022).