Morphology, Courtship and Mating of a Mixed Bilateral Gynander of Osmia ribifloris biedermannii Michener (Hymenoptera: Megachilidae)

Abstract

On rare occasions (,0.001%), newly fertilized eggs of solitary bees will produce two distinct lines of embryonic cells, one diploid and the other haploid. Due to haplodiploidy, these embryos as adults often appear half-sided or they have scattered patches of male and female secondary sexual traits (Drescher and Rothenbuhler, 1963; Jallon and Hotta, 1979; Akre et al., 1982; Kinomura and Yamauchi, 1994; Lucia et al., 2009). These intersexual bees often have a bilateral head and thorax and an abdomen that is primarily male or female (Gonzalez, 2004). These rare phenotypes known as gynandromorphs or gynanders are described for more than 60 bee species that are solitary or primitively social, half of which come from the Megachilidae, but are certainly more widespread than currently recognized, especially when gynanders result from being stylopized (Wcislo et al., 2004). Although gynanders and other genetic mosaics occur in all major bee families, opportunities have seldom arisen to study their habits when they are alive; and the effect of their mosaic appearance on courtship with wild-type nestmates has never been reported, until now. From the few reports of other arthropods that we can find, gynander courtship is surprising autonomous of its androgynous form. Gynanders of moths, fruit flies and locusts, for example, commonly express a unisexual gender based on their mate’s courtship displays, and it strongly correlates with the phenotype of the abdomen rather than with the phenotype of the head or thorax (Chaudhuri, et al., 1992; Maeno and Tanaka, 2007). Consequently, a gynander, even one with a half-sided head and brain, will behave either as a male or as a female, but rarely as both. Such a bee therefore cannot be simply dismissed as a sterile genetic freak; a few gynandromorphs of other arthropods reportedly live long enough to mate and raise young of their own. Gynanders of fruit flies and locusts, for instance, actively seek mates of one sex or the other (Jallon and Hotta, 1979; Yamamoto et al., 1998; Maeno and Tanaka, 2007). Lobster gynanders go a step further, raising healthy wild-type larvae from roes containing half the normal number of eggs (Syslo and Hughes, 1981; Cowan, 2000). Might solitary bee gynanders do the same? For the first time, we explore whether a single gynander will mate with wild-type males and females of a managed orchard bee Osmia ribifloris biedermannii Michener (Hymenoptera: Megachilidae). We report unusual aspects of the courtship displays of males that attempted to mate with our specimen. The gynander also briefly followed and antennated wild-type females, which are two odd non-sexual behaviors consistent with the ‘‘transvestism hypothesis’ proposed by Wcislo et al. (2004).