Developmental anomalies in Drosophila hybrids are apparently caused by loss of microchromosome.

Hybrids produced by crossing Drosophila virilis females to D. lummei males suffer from many developmental anomalies; the reciprocal hybridization yields normal offspring. Genetic analysis reveals that these anomalies involve a maternal effect: whether or not an individual will show an anomaly depends upon his mother's nuclear genotype. Several lines of evidence suggest that the proximal cause of the anomalies is the elimination of the D. lummei microchromosome (chromosome 6) from hybrids. Loss of the D. lummei microchromosome in this hybridization is known to involve a maternal effect (Evgen'ev, 1973), as mitosis in early development is under the control of maternally-acting genes.