Sex determination and larval sexual interaction in Bonellia viridis Rolando (Echiura : Bonelliidae)

Abstract

Sex determination in the echiuran Bonellia viridis Rolando has classically been regarded as depending primarily on the environment of the newly settled larvae. The majority of the sexually undifferentiated larvae settling on an adult female become males; the larvae which settle away from the adult female become females in most cases. Previous work on this problem is reviewed. The behaviour, including the time-course of settlement, and the development of the indifferent larvae of B. viridis in the presence and absence of an adult female is described. Evidence is provided that even in the absence of adult females there is an interaction between newly settled larvae such that up to 20% of larvae become attached to one another in pairs with masculinization of one partner, the other developing into a female. Using larvae cultured singly, it was shown that crude extracts of adult female proboscis and trunk body-wall and the pigmented secretion of an irritated female masculinize indifferent larvae, the vast majority of which would have developed into females in pure sea water. Solutions of the purified integumentary pigment, bonellin, gave inconsistent results. Our experiments prove conclusively that sex determination is metagamic, i.e., not fixed at fertilization but is the result of an interaction between genetic and environmental factors, in ~83% of all larvae. The main environmental factor is a substance produced by the female. The other 17% are larvae whose sex is determined exclusively by their genetic make-up. These are syngamic males, females and intersexes. The problem of indefinitely undifferentiated larvae is discussed.