Interspecific transplantation as a tool for indicating phylogenetic relationships.

Phylogenetic analysis has been widely accepted as the process by which the genetic continuity between populations of different species can be detected and therefore their genetic history traced. Such analyses have actually been based on a variety of well-recognized and defined characters. In the genus Drosophila, for example, external morphological characters, -4 internal morphological characters,5-7 and biochemical8 and cytological9-" characters have been used extensively. Interspecific ovarian transplantations were first introduced into such studies by Monod and Poulson'2 and by Vogt.'3' 14 Although these experiments, and especially those by Vogt,"4 demonstrated the potentiality of the technique for studies of evolutionary relationships, no other attempts have apparently been made. New techniques introduced since these early days of Drosophila work and accumulation of considerable information regarding gonadal development in insects encouraged the writer to carry out a large-scale study of interspecific ovarian transplantations. The main objective was to establish procedures that could be used to analyze phylogenetic relationships between species or species groups of the genus Drosophila, based on the degree of growth of transplanted ovaries. The secondary aim, no less important, was to determine the possibility of using interspecific ovarian transplantations for obtaining hybrids between distantly related species of the genus Drosophila, not possible with normal laboratory matings. This would make possible new comparative cytological studies. Materials and Methods.-Laboratory stock cultures from collections of The University of Texas were used. Twenty species of the genus Drosophila, representing most of the species groups of the subgenus Drosophila, were analyzed in these studies. Two species, D. virilis, a relatively primitive one,7s 9 and D. mulleri, an advanced species, were used as hosts. Late third instar larvae of the same physiological age were used for the operations. Donor larvae were dissected in Waddington's Ringer solution (37.5 gm NaCl, 1.0 gm CaCl2, 0.5 gm HCl, 1.0 gm NaHCO3, and 9 liters of H20); one ovary was transplanted into each host larva. A modification of the method by Ephrussi and Beadle'5 was employed. Operated adult flies were dissected 20 days after eclosion and their three ovaries were prepared as Feulgen-stained whole-mounts according to the method of Whiting.'6 The developmental condition of each ovary was recorded separately using the terminology of King,17 scoring from zero development to stage 14, maximum development. Detailed information about the species involved in these studies and methods applicable to each are to be published elsewhere.'8 Results and Discussion.-Representative cases-of the developmental condition of ovaries implanted into D. virilis hosts are shown in Table 1. It can be seen in columns 4-6 that development of individual ovarioles is autonomous; there is no indication for synchronization in oogenesis.