Renner, K. J. | bioRxiv Science

bioRxiv2025

A common mechanism and multiple advantages of pigment loss underlie the convergent evolution of albinism in cave animals

The loss of pigmentation is a hallmark adaptation of cave-dwelling animals but the underlying mechanisms are poorly understood. This study investigates the cellular and biochemical basis of albinism, the loss of melanin pigment, which convergently evolved across a wide range of cave animals. Is albinism caused by the interruption of pigment synthesis or elimination of the pigment cell lineage? The results suggest that albinism evolved by a common mechanism, a block in the first step of the melanin biosynthesis pathway, the conversion of L-tyrosine to L-DOPA, in diverse albino cave animals ranging from annelids and mollusks to vertebrates. Pigment cells are conserved in all tested albino cave species and distributed in patterns resembling pigmentation in their close surface relatives. The cells capable of melanin synthesis when provided with L-DOPA substrate were detected at the sites of injuries in a cave annelid, cave mollusk, and cave teleost, suggesting roles in innate immunity during tissue repair, which may explain why pigment cells are conserved despite the loss of pigment production. Next, we focused on the longstanding issue of the evolutionary forces involved in the regression of pigmentation in cave animals and explored hypotheses of why losing pigmentation might be adaptive in cave environments. First, we tested the hypothesis that pigment regression may conserve energy, which is important for survival in the food-limited cave environment. Our results indicate that melanin synthesis has a marked energetic cost in the cavefish, Astyanax mexicanus. Lastly, we show that the disruption of melanin synthesis at its first biosynthetic step is correlated with increased dopamine levels in multiple depigmented cavefish populations, including populations that evolved albinism independently. This result supports a widespread tradeoff between the melanin and catecholamine synthesis pathways in A. mexicanus. We conclude that the interruption of the conversion of L-tyrosine to L-DOPA in the synthesis of melanin confers multiple advantages that could serve as targets of natural selection, supporting an adaptive hypothesis for the evolution of albinism in cave animals.

evolutionary biology↗

bioRxiv2018

Sex Differences in Aggression: Differential Roles of 5-HT2, Neuropeptide F and Tachykinin

Despite the conserved function of aggression across taxa in obtaining critical resources such as food and mates, serotonins (5-HT) modulatory role on aggressive behavior appears to be largely inhibitory for vertebrates but stimulatory for invertebrates. However, critical gaps exist in our knowledge of invertebrates that need to be addressed before definitively stating opposing roles for 5-HT and aggression. Specifically, the role of 5-HT receptor subtypes are largely unknown, as is the potential interactive role of 5-HT with other neurochemical systems known to play a critical role in aggression. Similarly, the influence of these systems in driving sex differences in aggressive behavior of invertebrates is not well understood. Here, we investigated these questions by employing complementary approaches in a novel invertebrate model of aggression, the stalk-eyed fly. A combination of altered social conditions, pharmacological manipulation and 5-HT2 receptor knockdown by siRNA revealed an inhibitory role of this receptor subtype on aggression. Additionally, we provide evidence for 5-HT2s involvement in regulating neuropeptide F activity, a suspected inhibitor of aggression. However, this function appears to be stage-specific, altering only the initiation stage of aggressive conflicts. Alternatively, pharmacologically increasing systemic concentrations of 5-HT significantly elevated the expression of the neuropeptide tachykinin, which did not affect contest initiation but instead promoted escalation via production of high intensity aggressive behaviors. Notably, these effects were limited solely to males, with female aggression and neuropeptide expression remaining unaltered by any manipulation that affected 5-HT. Together, these results demonstrate a more nuanced role for 5-HT in modulating aggression in invertebrates, revealing an important interactive role with neuropeptides that is more reminiscent of vertebrates. The sex-differences described here also provide valuable insight into the evolutionary contexts of this complex behavior.\n\nSignificance StatementSerotonins (5-HT) modulatory role in aggression is generally reported as inhibitory in vertebrates but stimulatory in invertebrates. Using a novel invertebrate model system, we provide evidence of common pathways of aggression at the 5-HT receptor subtype level as well as 5-HTs interactive role with other neurochemical systems namely neuropeptide F and tachykinin. Additionally, we found that these effects were sex-dependent as well as stage-dependent affecting either the initiation or escalation stage of an aggressive contest. Our results reveal the impressive level of conservation with respect to neurochemical mechanisms among species as diverse as vertebrates and invertebrates, and highlights the need to consider multiple factors when determining potential taxonomic differences in how 5-HT mediates aggression.

neuroscience↗