10. Mar 2026

Developmental and molecular aspects of caste differentiation in the pharaoh ant, Monomorium pharaonis (L.)

Datum: 10. March 2026 | 15:00 – 16:00
Sprecher: Ruyan Li, University of Copenhagen
Veranstaltungsort: Central Bldg / O1 / Mondi 2a (I01.O1.008)
Sprache: Englisch

Hymenopteran social insects exhibit reproductive division of labor among adult females, where some individuals reproduce and obtain direct fitness, while others perform altruistic tasks that yield them indirect fitness benefits. In contrast to open society-forming lineages, where reproductive altruism is depends on environmental and social conditions so that adult females can switch between breeder and helper roles, the superorganismal ants, vespine wasps and crown-group corbiculate bees (bumblebees, stingless bees, honeybees) have evolved pre-imaginal caste differentiation via developmental canalization resulting in morphologically and behaviorally fixed and very different adult caste phenotypes, which are either reproductive gynes (future queens) or workers who express obligate reproductive altruism. Here, the queen and worker (sometimes soldier) caste phenotypes are best conceptualized as complementary colony-level analogues of the differentiated germline and soma cell lineages in metazoan bodies. This germline-soma analogy implies that lineages with caste-differentiated superorganismal colonies represent irreversible major transitions in evolution (MTEs), comparable to earlier lower-level MTEs to obligate eukaryotes multicellularity.

Unlike open society-forming lineages which lack irreversible pre-imaginal caste differentiation, superorganismal lineages have unequivocal reproductive division of labor with divergent developmental trajectories diverge across numerous functional traits for which gynes and workers differ. This differentiation normally begins at some point during larval development but in some species, most notably in the pharaoh ant Monomorium pharaonis, it starts already during embryogenesis in the egg stage. Using M. pharaonis as a model superorganism, this PhD project aimed to dissect the organ-specific developmental trajectories that characterize gyne and worker caste differentiation throughout immature development, and to elucidate the functions of key genes that orchestrate these differentiation processes.

In the first chapter, we demonstrate that caste differentiation is already pervasive across multiple organs in late embryos and first instar larvae of M. pharaonis, i.e. in developmental stages well before immature reproductives and workers become morphologically distinguishable in the second larval stage. Using established molecular markers, we documented complete germline degeneration in worker embryos and caste-biased development of the wing-disc primordium, fat body and nervous system. Through gene co-expression network analysis of published single-embryo transcriptomes, we experimentally validated the roles of ten gonad-specific genes whose expression robustly predicts whether a late embryo will develop as a gyne (gonads retained) or a worker (gonads degenerated). Strikingly, enrichment analysis of caste-biased genes in late embryos identified canonical metazoan germlinesoma specification pathways, indicating that deeply conserved developmental programs underpin germlinesoma differentiation at both the individual and colony levels.

In the second chapter, we show that, despite caste differentiation initiating very early during embryogenesis in M. pharaonis, canalized worker development can still be shifted toward a somatically functional gyne phenotype by juvenile hormone (JH)-mimic treatment of final (third) instar larvae. We showed that excess JH can activate gyne-biased development in workers across multiple organs – including the wing, eye-antennal and genital imaginal discs, flight muscles, fat body and brain. Developmental transcriptome profiling also uncovered a cohort of JH-sensitive genes that underpin these gyne-like somatic traits. However, ovary development remained unresponsive to JH treatment, corroborating an earlier finding that the worker germline is irreversibly eliminated during embryogenesis. These results indicate that caste-specific traits have modular, asynchronous JH-sensitivity windows – a pattern that may also govern the evolution of derived female castes in other ant lineages, such as permanently wingless (ergatoid) gynes and soldier caste phenotypes.

In the third chapter, we report the discovery and functionally characterization of gene sheng, a Hymenoptera-specific novel gene that acts as a pivotal regulator of gyne-worker caste differentiation in M. pharaonis. This gene has consistently gyne-biased expression across ant species during subsequent developmental stages, being expressed in multiple organs while ranking as the most strongly JH-responsive transcript identified. Knockdown of sheng in developing gynes reprogrammed their developmental trajectory toward a worker-like phenotype, with corresponding growth reductions in multiple organs, including wings, eyes, sperm-storage organ, key brain regions and the fat body. Mechanistically, Sheng operates downstream of JH and its Gce/Tai receptor complex, functioning as a transcription factor that translates systemic JH titers into caste-specific gene-regulatory programs. Chromatin profiling and transcriptomics identified numerous direct Sheng targets, including the developmental transcription factor spalt major, and revealed that Sheng modulates the Hippo signaling pathwaya universal regulator of organ size from insects to mammals. Finally, heterologous expression of sheng in the fruit fly Drosophila melanogaster and human cells enlarged organ size and increased proliferation, demonstrating growth-promoting activity in evolutionarily distant systems despite 700800 million years of divergence.

By integrating developmental anatomy, hormonal manipulation experiments and functional studies, this thesis uncovers key aspects of the regulatory mechanisms operating in pre-imaginal caste differentiation. The results clarify important genes that mediate morphological and physiological differences between castes, increasing our understanding of the gene regulatory networks that uniquely characterize the superorganismal ants.