A NOTE ON AN ANOMALOUS SEX-ZATI0 IN

DI~OS'OPHILA OBSO Ui2A

]3y CECIL GORDON

DeNa, r~men, t of Zodogy, U~iversity CdZege , Lobed, on

t~r fh8 course of inbreeding work of wild ferti]ize, d female D. obseu~'a gave in the F1 33 femMes and no males. At :E.rst sight this phenomelmn appeared to be the same as that deser.lbed by Gershenson (]928), and found, by him to be due to a gent. on the X-chrom.osome whirl, affected spermatogenesis (S{nrtevant & Dobzh~nsky, ]936) in such a way that the X sperms were in ~he main eliminated.

The females fhus obtained were crossed go males fl'om ,~n ordinary stock by paired matings, and of the 21 euRures so set. up only 8 were fertile as judged by the presence of larvae and pupae., while 7 yielded flies. This is less than the number of fertile cuRures usually obtained by paired matings in D. subobscura CollhL and, in view of the fact dm.t D. obscure is easier to cuRure than D. subobscz~,r~, this is probably sig.nificant. The resuRs are given in Table I.

TABLE I

Cui~m,e no . . . . 1 2 3 4 5 6 7

d~ 37 45 22 31 36 57 52 99 39 46 45 24 45 69 69

These animals were quite normal in appearance. Ib will be noticed that, in cu ra re 3 there is a disturbed sex-ratio

which is probably significant. To ~es~ for {he sex-ratio gent ~he males from this generation may be

crossed to any females, but at the s~me time i decided to havestigate the sex-ratio h: cuRm'e 3. Aeeord:ingly each of 18 males from cuRure 3 was crossed to one of its sisters, and each of 28 males from the other cuRures was crossed to a single female from the D. obscura stock, tn the even~ of the original effect being due to a sex-ratio gent we would expect one-half of the Iatter cultures to show the disturbed sex-rude, while the cultures from 3 would show a, disturbed sex-ratio more strongly.

The results obtained were entirely unexpected. Of th.e 18 cultures from 3 all were sterile, while of the 26 cultures from the other (out- crossed) group 2.3 were eompl.etely sterile, and 5, of which 3 yielded about

326 Av~,oma~ous Sez-9"al, io 4n Drosoph i l a , o b s c u r ~

20 flies of both sexes, showed a few pupae each. A mass mating of these lattei' was again sterits.

Ig is unfortuna,te tha t during the later stages of the investigation I was away from the labo~:atory owing to ilhess, and hence the reciprocal crosses in the P~ wm'e not made. As a consequence there is no obvious izlterprel?ation of' these results. The following considerations are offered tent atively:

(1) The _~: sex-ratio was due to a sex-ratio gene or some other genetic aberragion hitherto undescribed iu free-living animals, and subsequent events were independent of this and due to an inter-race cross of the t3q~e described in D. pseudoobsea~'ra (Lanoefield, ]929 ; Dobzhansky, 1936, etc.). This would mean ~hat in D. obscur<~ there exist :'aces whose presence has hither:to been undetected.

(2) I:faldane (1922) points out tha t s%rility of males and absence of females are probably degrees of the same phenomenon. Dobzhansky (1936) indicates tha t repeated out-crossing of hybrid females of D. pseudoobsG~rc~ go males of either of the races would, if not complicated by viability effects, restore the fertility of the male hybrids. Detlefsen (1914) showed that the mate hybrids in Cavie regained fm~tility with successive generations of back-crossing of originally hybrid females to males of one of the species. In this instance the events may have been as follo~°s. The original mating 'was between two races or subspecies of D. obsc~,'a and resulted in the absence of males and fertility of some of the females. These females o~ being crossed to one of the parental races which now eon~ribut.es more genetic material to the off'spring than the o~her race, produces viable but predominantly sterile males and females which were not tested. The probable intermediate position in relagion to some other types of hybridi.zation in DrosophiZa is set out :n Table II .

TABLE II

'*~r~elanogasl.cr o x simulans ~ ~ 9. 0 ~b~ent Smira~rZa (~ >: p~eudoobscv,ra $ J sterile, c~ oh,sou-re unl~:mvn x oh*cure slack ~9 fertile, c~ absent pseugoobscura :'ace A × race B 0. 9 fc:'til~, c~ present, but sterile

* ,~turtevang (1929). 1" Dobzhansky (1935).

This would indicate a type of hybridization hitherto undeseribed in D~'osopt~igc~.

(3) The phenon~enon appears different to that found, in Soiara co2¢oThih~, (Metz & Schmuck, 1931),where there are also unisexual broods. In Nsia.~a, although the mechanism is not thoroughly understood, nni- sexual broods are continuous; tha t is in all generations females prod~aee

C~CIL G O g D O N 327

either comp].etely male broods or completely female broods. Half the female offspring in any one culture in turn produce females only, while the other half produce males only, and this is no~ associated with par- thenogenesis. The males when produced are fertile. The relation of tlhis ~o the cytological behaviom' is obscure, bu~ it is clear tha t the cytological begaviour differs f rom t h a t of the Drosophigae.

(4) The possibilRy of a cytoplasmic inheri tance is no~ excluded. Since however the knowledge of cytoplasmic inheritance is ~:ather h~- complete, as are the data in this case, fur ther speculation is unprofitable.

N y thanks are due ~o Prof. R. C, P u n n e t t for helpful suggestions.

I%E]?EI-gENCES

DI~TL]~FS]~, J- A. (19t4). "(~enetic studies on a oa~-y species cross." 2~tbl,. ~arnsg. t~stn, No. 205.

Do~zu-~SKY, Tm (1935). "D.r~sophiIa ~z6anda, ~ new species." (_4~ne~ics, 20, 377- 91,

~ - - (1936)- "Studies on hybrid sterili/oy. II. Localis~tion of sterLlity fRctors in Droso&Mlapseudoobscura hybrids." Ibid. 21, 113-36.

G:~sr~:E~so_~, S. (1928). "A new sex-ratio abnormality in 2)~'osophila obscure." Ibid. 13, 4S8-507.

Fr~DAXX, J. B. S- (1922). "Sex ratio and unisexual sterility in hybrid animals." J. Goner. 12, 101-9.

LA~O~IELD, I). E. (1929). "A genetic study of crosses of two races or physiological species of Drosq~)hik~ obscu~.a." Z. indukt. Abstc~mwz.- u. Va'srbLehre, 52,287-317.

I~ETZ, CH.~mL~S W. & Scm~mc~, M. Loms~ (193I). "Studies on sex determination and the sex chromosome mechanism in Sciara." Get,sties, i6,225-53.

S~m~T~VaN'r, A. It. (1929). "The genetics of .Drosophila si~eglc~zs." Publ. Carncg. I~zstn, 399, 1-62.

STm~¢EV~, A. K. & Do~zm~5"sK'~L TK. (1936). "Gcographiea.] distribution and cytology of "sex-ratio ' h~ Droso2hila pscudoobscura and related elastics." Genetics, 2 t , 4~73-90.