Early social conditions affect female fecundity in hermaphrodites

Social conditions experienced prior to sexual maturity influence reproduction later in life in many animals. In simulta- neous hermaphrodites, variation in mating group size influences reproductive investment. As the mating group size increases, re- productive resources devoted to the female function decrease in favor of the male function. Prior to sexual maturity, many her- maphrodites have a protandrous phase during which they produce sperm and can fertilize hermaphrodites＇ eggs. In the simulta- neously hermaphroditic polychaete worm Ophryotrocha diadema, the cost of male reproduction during adolescence is spread over the whole energy budget of worms as shown by a reduced growth rate, a delayed age at sexual maturity and the shortening of life span compared to protandrous males that do not reproduce. Little is known on whether social conditions experienced dur- ing development affect reproductive investment of immature individuals. We investigated whether social conditions affected the length of the protandrous phase, body size and also the subsequent female fecundity of same-age protandrous individuals of O. diadema, which did not had to face competition for egg fertilization. Results show that in large group sizes protandrous males lengthened their protandrous phase, slowed down body growth and decreased their individual investment at the first egg laying compared to protandrous males that were reared in isolation. In the successive egg layings worms adjusted their egg output to the current social conditions. We interpreted these results as an indication that early social conditions represent a social stress result- ing in a reduction of the overall reproductive resources up to the first egg laying .

Increased population density reduces body growth and female investment in a simultaneous hermaphrodite

Sex allocation theory applied to hermaphrodites assumes that there is a trade off between the alloca- tion of resources to male and female functions, within a fixed reproductive resource budget. Charnov＇s classic resource allocation model predicts a more female-biased sex allocation when competition among different sperm donors is low due to diminishing fitness returns for male investment. By manipulating the social group size, one automatically changes the population density at which individ- uals live. Increasing population density may affect reproductive allocation, leading to resource compe- tition and/or to increased concentration of harmful metabolites. This could lead to an over- or under- estimation of the individual adjustment of sex allocation responses to mating opportunities. In this article, we tested the effects of density and social group size separately on female investment and body growth （considered as proxy of the overall energy budget） in the simultaneously hermaphroditic polychaete worm Ophryotrocha diadema. We manipulated social group size （i.e., monogamous and promiscuous regimes） and density （i.e., 4 levels） using a full-factorial design, to identify the underlying factor affecting female allocation （in terms of egg production） and body growth. In contrast to findings of previous experiments, we found that an increase in population density reduced body growth and egg production of hermaphrodites irrespective of social group size. We advance the hypothesis that the increase of catabolites and oxygen consumption in high-density conditions reduces the overall resource budget and this could obscure group size effects on female fecundity.

Discovery of Gynoecium Color Polymorphism in an Aquatic Plant

Flower color polymorphism exhibited by natural populations provides an opportunity for understanding the evolutionary mechanisms contributing to the diversity of floral morphology. However, little is known about the color polymorphism of female organs in flowering plants. Here we report gynoecium color polymorphism in Butornus urnbellatus （Butomaceae）, an emergent, aquatic monocot. Populations from Mishan, northeastern China comprised two morphs; gynoecia are either pink, as observed in other areas, or white. We measured floral traits and female fecundity in the two gynoecium color morphs in the field. There was no significant difference in plant height, pedicel length, and flower size including petal, sepal and gynoecium between the two morphs, but plants with pink gynoecia had wider inflorescence stalks, larger inner whorl anthers and produced more pollen and ovules than those with white gynoecia. Correspondingly, we found that seed production was significantly higher in the pink than in the white morph. This new finding suggested selection against white gynoecia in part because of low fecundity, consistent with the rarity of the white gynoecium morph in this species.