Intersexuality incidence, sex ratio fluctuations and intersex reproductive output as factors affecting the temporal variation of intersexed populations of the marine amphipod

Irene Martins , Alex T. Ford , Teresa F. Fernandes

The occurrence of intersexuality in populations of the marine amphipod has been related to several causes, including anthropogenic contamination, microsporidian parasites and environmental sex determination (ESD). At the individual level, the costs of intersexuality in are reflected in reduced reproductive output of intersexes compared to normal specimens. To understand the consequences of intersexuality at the population level is essential to fully evaluate the impacts of intersexuality and other reproductive disorders in nature. The aim of the present work is to simulate the density variation of real populations of with different intersexuality incidence, proportions of phenotypes and intersex female reproductive rates. To achieve that, we have incorporated empirical data from three intersexed populations of into a population dynamic model. The original model was calibrated and validated for a stable population without intersexuality and sex ratio ∼ 1. In the present version, the model includes an extra group, intersex females, but the total sum of flow rates from juveniles to adult groups was kept the same as in the original model to respect the previous validation. In these circumstances, results indicate that the male-biased population is very sensitive to intersexuality, even at low incidence levels, and exhibits significant density decrease through time if conditions do not change. On the contrary, the two populations with a preponderance of female-biased sex ratios, low or moderate intersexuality incidence (up to an average value of 9% of the population) and with intersex reproductive rates corresponding to 80% the value of normal specimens, exhibit long-term persistence. The model is highly sensitive to adult female parameters (e.g. mortality), indicating that small variations on its values can significantly change population responses. Overall the present results suggest that, in spite the costs of intersexuality in , the temporal variation of intersexed populations seems to depend on intersex incidence level, sex ratio fluctuations and reproductive output of intersexes. Simulations also suggest that other population processes (e.g. mortality rates of normal females) may play important roles in the maintenance of populations with intersexuality.

Mots clés: Amphipod; intersexuality; reproductive disorders; reduced fitness; sex ratio; population-level effects; mathematical model

Irene Martins, Alex T. Ford, Teresa F. Fernandes. Intersexuality incidence, sex ratio fluctuations and intersex reproductive output as factors affecting the temporal variation of intersexed populations of the marine amphipod. Marine Environmental Research, 2009, 68 (4), pp.163. ⟨10.1016/j.marenvres.2009.06.007⟩. ⟨hal-00505737⟩