Section 9 — Population Genetics 



Rabaul is involved the F2 does not come through. 

 However, backcrossing of the Fi female to males 

 from the original parents is successful in these 

 cases. 



Cytologically the inversion complex ORE has 

 only been detected at Rabaul and is homozygous 

 there. Apart from IIRE only simple HID with a 

 frequency of 56 per cent has been detected. 



Thus on the available evidence we may 

 distinguish two races of D. rubida between which 

 there is considerable sexual isolation and hybrid 

 sterility: Race A from northern Queensland and 

 Eastern New Guinea without inversion IIRE and 

 Race B from Rabaul with inversion IIRE. 



9.56. The Relation between Fitness Components and 

 Equilibrium Frequencies in an Experimental 

 Population of Drosophila melanogaster. Ray 



Moree (Pullman, U.S.A.). 



In laboratory populations initiated with 

 equal frequencies of the alleles + and e", the 

 carriers of e" \e" fell rapidly to a mean stable 

 frequency of about 2.46 per cent. That similar 

 results obtain with several alleles of e and that 

 stabilization is probably a consequence of the 

 superiority of the carriers of +/e", is well known. 

 Several fitness components possibly contributing 

 to these effects were examined. Mass mating 

 tests (but not choice tests) indicated only slight 

 selection against e"/e" . Viability tests, with 

 crowding such that numbers of parents and 

 offspring were approximately equal., gave 

 coefficients of 92, 100 and 46 per cent for the 

 carriers of + / + , + /e" and e"/e", respectively 

 (Moree and King, 1961). Fecundity tests, 

 corrected for inviability, gave coefficients, in the 

 same order, of 88, 100 and 85 per cent (Moree, 

 1 962). Since such components measure sequential 

 events they may be multiplied to obtain partial 

 fitness coefficients of 81, 100 and 41 per cent 

 (same order). These coefficients indicate an 

 equilibrium frequency of q (e") = 24.28 per cent. 

 If panmixis is provisionally assumed, then 

 q 2 (e"/e") = 5.9§ per cent. But this is an over- 

 estimate. If the above viability coefficients are 

 included the calculated equilibrium frequency of 

 e"/e" is reduced to 3.09 per cent, since, at 

 equilibrium, inviability changes genotype, but not 

 gene, frequencies. By t-test, 2.46 and 3.09 percent 

 differ (P<0.01, 7V$j 7000). Thus while heterosis 

 accounts for stabilization, and inviability and 

 infecundity for about 97 per cent of total se- 

 lection, a small fitness component, probably 

 related to selective mating, is yet to be accounted 

 for. 



9.27. "Yellowish" in Drosophila bifasciata, con- 

 ceivable as an Incipient Species. Daigoro 

 Moriwaki and Osamu Kitagawa (Tokyo, 

 Japan). 



In northern parts of Japan, e.g. Hokkaido, 

 Drosophila bifasciata which is known to inhabit 

 cold or mountainous districts in the rest of the 

 country can be collected at level ground even in 

 early summer. At three localities, so far confined 

 to Hokkaido, some mutant flies with yellowish 

 thorax color — designated by "yellowish" (yli)— 

 were collected once in a while. Interestingly, they 

 were found always together with wild type flies. 



Besides the thorax color, "yellowish" has one 

 — not two — black stripe on the scutum, which 

 expands postlaterally forming a trident-like 

 pattern and it also shows some differences in 

 metric characters from the wild type. 



"Yellowish" females can cross with wild type 

 males to some extent, while the reciprocal cross 

 is difficult. Isolation index obtained with the 

 aid of "male-choice" method, amounts to 0.8 on 

 the average. It seems that "yellowish" is consider- 

 ably isolated sexually from the wild type, and 

 can live sympatrically with it. 



According to the test for fertility of the Fi 

 hybrid flies, the male hybrids are always sterile, 

 notwithstanding slight fertility of the female 

 ones. 



On examining salivary gland chromosomes 

 of the hybrid between yh and +, no complete 

 paring has been seen in any pair of the homolo- 

 gous chromosomes. Although further careful 

 comparisons are being made, it is tempting to 

 speculate that "yellowish" may be an incipient 

 species. 



9.58. A Study of the Genetic Structure of Natural 

 Populations by Means of Polygenic Viability 

 Mutation Rate in Drosophila melanogaster. 



Terumi Mukai (Misima, Japan). 



A single male Pm/ + from a cross of Cy/Pm x 

 W160 (Burdick's isogenic wild-type stock) was 

 sampled and multiplied to 104 by crossing to 

 Cy/Pm females, and each wild-type second 

 chromosome has been maintained through a 

 single male by the cross of Cy/Pm (5$$)xPm/ 



+ 7 Oc?) (*=1. ■ • • » 104 ) f° r tne Purpose of 

 accumulating spontaneous mutations affecting 

 viability. 



In generations 10, 15, 20, and 25, the homo- 

 zygous viabilities of those 104 chromosomes were 

 estimated by Cv-method (Wallace, 1956) and 

 their increasing rate of genetic variance and 



162 



