Fecundity and fertility in female homo- and heterokaryotypes of the tumorous-head strain of Drosophila melanogaster.

HE tumorous-head (tu-h) strain of Drosophila melanogaster has been the T subject of extensive investigations (for a review of the literature see CrARDNER 1959; WOOLF, KNOWLES and JARVIS 1964). The expression of the trait depends on a semi-dominant gene at position 56 on the third chromosome and a sex-linked recessive gene controlling a maternal effect. Modifiers affecting penetrance have been detected (GARDNER and GARDNER 1955). Two types of third chromosome exist in the strain-the 3A chromosome, containing the tu-3 gene, and the 3B chromosome containing, in addition to the tu-3 allele, the recessive gene for scarlet eyes ( s t ) and the Payne Inversion ( I n ( 3 L ) P ) . The 3B/3B combination is lethal (WOOLF and PHELPS 1960). Male and female homokaryotypes (3A/3A) are less viable than heterokaryotypes (3A/3B). Female homokaryotypes produce a significantly lower number of off spring than female heterokaryotypes, but no fertility difference exists between the two karyotypes in males (WOOLF and CHURCH 1963; WOOLF and KNOWLES 1964). Balanced chromosomal polymorphism, preserved by selection for inversion heterokaryotypes, is common in Drosophila populations. The reproductive advantage of heterokaryotypes has been assumed to be associated with the chromosome pair in which the gene arrangement occurs (PAVLOVSKY and DOBZHANSKY 1966), although recent studies indicate that interaction between the heterokaryotype and the genetic background may also be involved (WOOLF and KNOWLES 1964; WOOLF, KNOWLES and JARVIS 1964; SPIESS and SPIESS 1966). The purpose of lhis investigation was to ascertain if the greater productivity of tu-h female heterokaryotypes is due to greater fecundity (the number of eggs produced) and/or greater fertility (the percentage of eggs reaching the adult stage) (definitions after MAYR 1963). A second aim was to determine the chromosomal control of these parameters to find if the genetic background is involved in the maintenance of this balanced polymorphism. Thirdly, evolutionary divergence of the control of these parameters could be assessed by use of tu-h strains which have been separated for over 16 years.