Eli D. Aflalo

Isolation and characterization of a female-specific DNA marker in the giant freshwater prawn Macrobrachium rosenbergii

In this study, a female-specific DNA marker in the freshwater prawn Macrobrachium rosenbergii was identified through amplified fragment length polymorphism (AFLP). The AFLP-derived sequence-characterized amplified region (SCAR) marker was tested in over 200 individuals, giving reproducible sex identification. Further molecular characterization of the sex-markers genomic region (∼3 kb long) revealed the presence of tandem and inverted repeats. The ∼3-kb sequence was identified both in male and female prawns, but with subtle differences: a deletion of 3 bp (present in female prawn but absent in male prawn) identified upstream of the SCAR marker sequence and two female-specific single-nucleotide polymorphisms, both indicating that male prawns are homozygous, whereas female prawns are heterozygous in this locus. Fluorescent in situ hybridization showed the ∼3-kb sequence to be unique: to the best of our knowledge, this is the first report of a unique sex-specific sequence observed in situ in crustaceans. The sex-specific marker identified in M. rosenbergii may have considerable applied merit for crustacean culture in that it will enable the determination of genetic sex at early developmental stages when phenotypic differences are not identifiable.

Experimental evidence of a sex reversal process in the shrimp Hippolyte inermis

Summary Hippolyte inermis Leach 1815 is a shrimp which forms stable populations in seagrass meadows of the Mediterranean Sea and along the Atlantic coast of Spain. Investigations from the last century have demonstrated specimens experiencing a male stage prior to switching to females (i.e., protandric sex reversal). Further studies have demonstrated that not all females are derived from sex reversal: young females apparently deriving from direct differentiation are present in natural populations. In recent years some authors have claimed that the species is simply gonochoristic, mainly based on the absence of ovotestis development. In order to establish if the species is a peculiar hermaphrodite or a simple gonochoristic, H. inermis postlarvae were individually cultured in Petri dishes in a semi-closed system. Their exuviae were regularly collected, fixed and stained, to monitor the sex and the size of each individual over time. In addition, histological sections were examined and we observed an individual who lost its appendix masculine and developed an active ovary. In contrast, specimens that retained their appendices masculinae exhibited a male reproductive system. Fourteen such individuals who lost their appendices masculinae were observed throughout the experiment, suggesting a mechanism of sex reversal in decapod crustaceans in which an ovotestis may be absent during the transition to the opposite sex.