Chemical induction of mixis in the rotifer Synchaeta tremula

Abstract

In rotifers of the genus Brachionus, a chemical factor that accumulates during population crowding is necessary and sufficient to trigger sexual reproduction (mixis). In other monogonont rotifer species, field data and some laboratory studies indicate that mixis is density-dependent; however, to date it is unknown whether this reaction is chemically mediated as in Brachionus. Here we provide experimental evidence that mixis in the rotifer Synchaeta tremula is both density-dependent and chemically mediated. S. tremula cultured at high population densities (10 individual mL−1) produced 15.4% mictic offspring, whereas those cultured at low population densities (0.25 individual mL−1) produced only 0.3% mictic offspring. Conditioned water isolated from exponentially growing mass cultures induced significantly higher levels of mixis in the offspring of individually cultured females, when compared with untreated control medium (15.7 versus 1.4% mixis). In S. tremula, the propensity of females to respond to the mixis chemical decreased strongly with age. The highest proportion of mictic offspring (up to 63%) was produced by females of the youngest adult age class (∼24–48 h old). Females older than 3 days were virtually unresponsive to the mixis stimulus.