GENOME ORGANIZATION IN THE HESSIAN FLY, MAYETIOLA DESTRUCTOR (SAY) (DIPTERA: CECIDOMYIIDAE)

The chromosome cycle in the Hessian fly, Mayetiola destructor (Say), is described. Chromosome numbers and morphologies in salivary gland, neural ganglion, and germ line cells are characterized by orcein- and silver-staining and C-banding. A genetic model for sex determination is presented.; Larval salivary glands of the Hessian fly are composed of two regions. Four polytene chromosomes were identified in basal region cells. Nuclei in the filament region have a coincidence of polyploidy and polyteny. Morphogeneic analysis of the salivary glands indicated that the basal region is the probable source of plant growth-inhibiting substances produced during larval feeding. The filament region is active primarily during a nonfeeding period.; Karyotype analysis demonstrated that the somatic genome of the Hessian fly consists of two pairs of autosomes and two pairs of sex chromosomes. Female somatic cells are disomic for the autosomes and the sex chromosomes. Male somatic cells are disomic for the autosomes and monosomic for the sex chromosomes. Polytene and mitotic chromosome morphologies and indices were used to establish correlations between mitotic and polytene chromosomes.; The germ line genome consists of 2n = 8 S-chromosomes, which also are present in the soma, and 20 to 35 E-chromosomes, which are restricted to the germ line. During spermatogenesis n = 4 maternally-derived S-chromosomes are physically modified in the male germ line and are the only chromosomes transmitted in the sperm. During oogenesis, 2n = 8 S-chromosomes are modified in the female germ line and n = 4 S-chromosomes and a set of E-chromosomes are transmitted in each ovum. Except when sex-chromosome nondisjunction occurs during oogenesis, each zygote begins development with 2n = 8 S-chromosomes.; A genetic model involving three alleles at an autosomal locus was developed to explain the production of monogenous and bisexual progenies by Hessian fly females. Male development results when two sex chromosomes are eliminated from presumptive somatic cells during embryogenesis. Maternal genotype controls the elimination of the sex chromosomes and thus, conditions the sex of her progeny. Results were compared to other studies involving the Hessian fly and other cecidomyiids.