The genetics of mimetic colour polymorphism in the large narcissus bulb fly, Merodon equestris Fab. (Diptera: Syrphidae)

Abstract

Genetical studies have revealed six gene loci concerned with body hair colour in Merodon equestris . These are: (i) Bulborum . Darkens all the thorax including the scutellum except the anterior. The dark allele is dominant. (ii) The modifier U. In conjunction with bulborum this produces the morph subvalidus . This is characterized by some sexual dimorphism. These are certain sex differences in the degree of darkening of the second abdominal segment and the female shows a degree of darkening of the anterior thorax. The dominant dark allele only expresses itself when the dominant allele of bulborum is present. (iii) The modifier V. In conjunction with the dominant alleles of bulborum and the modifier U only the dominant allele of the modifier V gives the sexually dimorphic colour type validus . The second abdominal segment is more or less completely blackened and in the female only the anterior of the thorax is completely blackened. (iv) Equestris . The dominant allele produces a black thoracic band like bulborum except that the scutellum is coloured. (v) Transversalis . Expression is limited to the female, although the locus is inherited autosomally. There is a black band on the third abdominal segment. (vi) Ground colour. There are three alleles determining the distribution of orange and yellow over the thorax and abdomen. They are YOYO, OOYY and OOOO, the letters referring to yellow or orange coloration from the anterior of the thorax to the abdomen tip. The dominance is simple: YOYO is dominant to OOYY and OOOO and OOYY is dominant to OOOO. Within the colour types YOYO and OOYY there are colour variants YYYO, YOYY, YYYY and OOOY, OOYO respectively. Orange is dominant to yellow in the YOYO category and yellow dominant to orange in the OOYY category. The OOOO can be regarded as a universal recessive and can be treated as a separate category or as a variant of the OOYY category. Linkage has been detected between three of the loci. These are equestris, bulborum and ground colour and the order of mention is the order of linkage on the chromosome. Linkage, however, is not very strong so that some recombination occurs. Some aspects of the population genetics of the colour polymorphism have been studied using such little data as are available. Linkage disequilibrium has been found in the field for the linked genes bulborum and ground colour appears to be strongly maintained. A listing of potential models for the 34 colour types of M. equestris reveals that the colour types most commonly occurring, and maintained at high frequency and in linkage disequilibrium, are the best mimics of bumble bees in the United Kingdom. Some aspects of the evolution of the colour polymorphism in Europe, particularly in relation to the related species M. flavus are discussed. The chromosome number found confirms that found by other workers (2 n = 12) and polytene chromosomes have been demonstrated in a variety of tissues