Early social conditions affect female fecundity in hermaphrodites

Male larval experience of cues from adult rivals alters lifetime sperm investment patterns in a sperm heteromorphic moth, Ephestia kuehniella

Male animals may adjust their resource allocations for reproduction and other fitness functions in response to cues from rivals. For instance, adult males increase their investment in sperm for a higher paternity share when they perceive sperm competition risk in their surroundings. In nature, both juveniles and adults may coexist spatially and temporally. Yet, it is not clear how juvenile males of different ages respond to cues from adult rivals and fine-tune their lifetime investment in sperm production and ejaculation in any insect. Here we used the Mediterranean flour moth, Ephestia kuehniella, which produces both fertile eupyrene and infertile apyrene sperm, to explore this question. We demonstrate that the late, but not early, instar larvae are sensitive to adult male cues. As a response, they produce more sperm before emergence and their resultant adults have shorter mating latency and ejaculate more sperm in the first few matings. When the juvenile stage produces more eupyrenes, the adult stops making these sperm, but regardless of the number of apyrenes produced during the juvenile stage, the adult continues to make them. These findings suggest that the number of spermatogonia for eupyrenes may be limited and that for apyrenes may be flexible. Our results show that the insect does not trade off survival, mating frequency, body size, or testis size for sperm production in response to adult males during the larval stage. Knowledge created in the present study offers insight into the stage-dependent sensitivity of juvenile males to cues from adult rivals and subsequent lifetime resource allocations.

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 allocation 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 individuals live. Increasing population density may affect reproductive allocation, leading to resource competition and/or to increased concentration of harmful metabolites. This could lead to an over- or underestimation 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.

De novo transcriptome sequencing of a non-model polychaete species

Transcriptome sequencing is a useful method for studying gene-sequences data, especially in non-model organisms whose genomic sequences are yet to be determined. Indeed, even without any genome reference, transcripts can be assembled de novo to produce a genome-scale transcription map. Here we describe a de novo transcriptome assembly for the polychaete worm Ophryotrocha diadema, generated from 454-sequencing (Roche GS-FLX Titanium Sequencing). We describe the sequencing, the assembly and functional annotation of EST sequences, and the level of transcriptome coverage provided by our sequence data. The sequences we assembled represent a wide depiction of expressed genes from throughout the lifespan of O. diadema. We found gene sequences to be part of a broad range of functions (i.e. biological and metabolic processes, immune system, reproductive processes, development and growth, signalling and response to stimuli) and we therefore built the first transcriptomic reference for Ophryotrocha polychaete worms. These results encourage us to focus our investigation on many aspects of O. diadema and to extend our research to co-generic species.