Maladaptive Sex Ratio Adjustment in the Invasive Brine Shrimp Artemia franciscana

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English. Sex allocation theory is often hailed as the most successful area of evolutionary theory due to its striking success as a predictor of empirical observations [1]. Most naturally occurring sex ratios can be explained by the principle of equal investment in the sexes [2–4] or by cases of “extraordinary” sex allocation [5]. Deviations from the expected sex ratio are often correlated with weak selection or low environmental predictability (e.g., [6, 7]); true cases of aberrant sex allocation are surprisingly rare [8]. Here, we present a case of long-lasting maladaptive sex allocation, which we discovered in invasive populations of the exclusively sexual brine shrimp Artemia franciscana. A. franciscana was introduced to Southern France roughly 500 generations ago [9]; since then, it has coexisted with the native asexual species Artemia parthenogenetica [10]. Although we expect A. franciscana to produce balanced offspring sex ratios, we regularly observed extremely male-biased sex ratios in invasive A. franciscana, which were significantly correlated to the proportion of asexuals in the overall population. We experimentally proved that both invasive- and native-range A. franciscana overproduced sons when exposed to excess females, without distinguishing between conspecific and asexual females. We conclude that A. franciscana adjust their offspring sex ratio in function of the adult sex ratio but are information limited in the presence of asexual females. Their facultative adjustment trait, which is presumably adaptive in their native range, has thus become maladaptive in the invasive range where asexuals occur. Despite this, it has persisted unchanged for hundreds of generations.