ALBERT

Description: In many organisms, offspring fitness is markedly affected by size at hatching. Theoretical models predict offspring from one clutch to have the same optimal size, but empirical studies report within-clutch variation in offspring sizes for a wide range of species. According to the differential allocation hypothesis, this could be explained, at least in part, by multiply mated females selectively provisioning their offspring depending on the sperm donor. We tested this hypothesis in the internally fertilising, colonial sea squirt Diplosoma listerianum, whose larvae are non-feeding so that females’ provisioning of oocytes provides all of the energy for larval dispersal and metamorphosis. We show that there is high within-clutch variation in offspring size in a natural D. listerianum population. When we then crossed laboratory clones pairwise in a fully factorial design in the laboratory, we found that the same acting female clone produced differently sized oocytes with different mating partners. This result does not reflect the effect of direct male benefits, as the water-borne sperm cell is the only contact between males and females, but nevertheless indicates that females allocate resources to their offspring in part influenced by the father’s identity. This pattern would be expected to contribute to within-clutch variation in offspring size in mixed-paternity broods.

Description: Organisms and their resident microbial communities form a complex and mostly stable ecosystem. It is known that the specific composition and abundance of certain bacterial species affect host health and fitness, but the processes that lead to these microbial patterns are unknown. We investigate this by deconstructing the simple microbiome of the freshwater polyp Hydra. We contrast the performance of its two main bacterial associates, Curvibacter and Duganella, on germfree hosts with two in vitro environments over time. We show that interactions within the microbiome but also the host environment lead to the observed species frequencies and abundances. More specifically, we find that both microbial species can only stably coexist in the host environment, whereas Duganella outcompetes Curvibacter in both in vitro environments irrespective of initial starting frequencies. While Duganella seems to benefit through secretions of Curvibacter, its competitive effect on Curvibacter depends upon direct contact. The competition might potentially be mitigated through the spatial distribution of the two microbial species on the host, which would explain why both species stably coexist on the host. Interestingly, the relative abundances of both species on the host do not match the relative abundances reported previously nor the overall microbiome carrying capacity as reported in this study. Both observations indicate that rare microbial community members might be relevant for achieving the native community composition and carrying capacity. Our study highlights that for dissecting microbial interactions the specific environmental conditions need to be replicated, a goal difficult to achieve with in vitro systems.