466 Genetics of Sex Determination 



females; without symbionts, into males. Schrader's work in related 

 species shows there is no 2X-1X mechanism present, and thus a F/M 

 balance of the monoecic type may be expected. The change of metab- 

 olism by presence or absence of the symbionts provides the interior 

 milieu for the change of F or M action according to its inherited norm 

 of reaction with the two different environments. All former discussions 

 of this situation apply also to this extreme case. 



B. MULTIFACTORIAL SEX DETERMINATION? 



Another unconventional type of sex determination (we call the 

 standard F/M balance mechanism the conventional one) has been 

 claimed for isopods. In diploid species, Vandel (1938-1947) found 

 the existence of females which produce both sexes among their off- 

 spring, others with only daughters, and still others with only sons. As 

 the father does not seem to affect this result, Vandel concludes that 

 the females are XY and that directed maturation divisions occur, just 

 as we discussed above as a possibility for Dinophilus. In addition, a 

 cytoplasmic factor controlling the directed meiosis is assumed, since 

 the monogenic behavior (only sons or daughters) is inherited without 

 participation of the father. Actually, an XY or WZ group was never 

 found cytologically. A tendency to hermaphroditism is also present in 

 the isopod groups. A priori, one would expect Vandel's explanation or 

 a similar one to be correct, because neither total nor partial monoecists 

 or intersexes occur in the crosses, but only the pure sexes in ratios 

 from more or less normal up to extreme (100:0), and because some 

 constitution of the mother is responsible. Since we know that all these 

 conditions occur in aphids with a proved X-chromosome mechanism 

 plus cytoplasmic controlling factors, we need not look for another type 

 of explanation. But de Lattin (1952) has studied some other genera of 

 oniscoids and asserts that he has disproved Vandel's explanation and 

 substitutes for it a system of "multifactorial sex determination," de- 

 rived from Kosswig's interpretation in fishes. Since I consider the entire 

 idea of multifactorial sex determination a fallacy (see IV 2), except 

 when it is only an unfortunate term for describing modifiers of the 

 action of the F/M balance, more details must be discussed. 



Vandel called Trichoniscus females with ordinary offspring in the 

 ratio 1 9 : 1 (J the "amphogenes," and different types of females with 

 offspring either wholly or partially of one sex, the monogenes. The 

 latter fell into six groups: (1) complete arrhenogenes, producing only 

 sons; (2) incomplete arrhenogenes, with a few exceptional daughters; 

 (3) complete thelygenes, giving only daughters; (4) incomplete thely- 



