A Minute-Like III-Chromosome Recessive in Drosophila Melanogaster

After his discovery of this first Minute mutation, Bridges within a year found no less than about fifty minute-bristle mutant characters, and it was apparent that every one of this series was more or less regularly associated with the small bristles. Genetical examination proved that one particular type of Minute was recurring and constituted the bulk of the cases. This type, called Diminished, was by Bridges (1921) demonstrated to be due to the absence of one member of the IV-chromosome pair. The rest were, despite their outward similarity in many details, found to have very different loci in the chromosomes. Of those so far described, four had their loci in different parts of the III-chromosome (Bridges and Morgan, 1923), two are included in the II-chromosome map (Morgan, Sturtevant, and Bridges, 1921), and in one case the character complex depended on the presence of two genes, one in the second and one in the third chromosome linkage group.

All these mutants differed from Diminished and from each other mainly in the degree of development of the several common characteristics mentioned above. To these characteristics should also be added a tendency to roughening of the eyes and to reduction of the aristae.

All the Minutes so far mentioned are genetically dominant. Judging from general statements of Morgan, Sturtevant, and Bridges (1920), and of Bridges and Morgan (1923), this is true of all the typical Minute characters, as is also the fact that they are lethal when homozygous. In the description of the “Minute epidemic” it is, however, mentioned that “also a few recessive small-bristle mutants were found and recovered in F₂, notably tiny bristles” (Bridges and Morgan, 1923). It is not apparent from the data so far published, to which extent these recessive small-bristle mutants exhibit the aggregate of multiple effects typical of the dominant Minute characters. The II-chromosome recessive “morula” (Bridges and Morgan, 1919), the principal characteristic of which is the roughened, moruloid eye-surface, has also bristles which are markedly reduced in size. The latter peculiarity is not quite constant, since about 10 per cent, of the homozygous flies have bristles of normal size. The morula females are sterile when mated with males of their own race. When mated with males of other races morula females in a few cases produced a very small number of individuals (Lynch, 1919). The viability is excellent. Nothing about delayed hatching or about other “Minute-characteristics” is mentioned in the description. In the recessive II-chromosome mutant “reduced-bristle” studied by Miss Lynch (1919), the bristles on the thorax are reduced either in size or number. Here, too, the bristle alteration is associated with markedly lowered fertility. No other peculiarities are described. The III-chromosome recessive “spineless” in which all the bristles are greatly reduced in size, exhibits none of the other peculiarities of the small-bristle dominants.

Thus, one might get the impression that the typical aggregate of minute-characteristics is restricted to a particular class of mutations, which are genetically dominant and lethal when homozygous. In this paper is given a short account of a recessive gene which in every other respect strikingly exhibits the features typical of the dominant Minute character complex.

Were it not for the recessiveness, the description given by Bridges of his first Minute might equally apply to this mutation.

The first appearance of the character, called “short-bristle” (sb), was in a culture in which three fat ^* females, derived from an F₂ involving the Il-chromosome recessives fat and dumpy, were tested by dumpy males from stock (Culture 3386, 3rd March 1924). This culture gave during the first five days 110 wild-type flies and 8, males and females, in which all the bristles were much more slender and considerably shorter than normal Short-bristle individuals of the same type were later found in the dumpy stock bottle, so it is apparent that the mutative change which gave rise to the new character must have occurred in this stock, and that some of the flies used in the test mentioned were heterozygous for the short-bristle character.

Since from an external examination it seemed possible that we were dealing with a reappearance of the III-chromosome recessive spineless or with an allelomorph of this gene, some of the short-bristle females obtained were crossed to spineless (ss) glass (gl) males from the ss gl stock bottle. The test (3431) gave only wild-type offspring, thus demonstrating that the alteration mentioned could not be due to the spineless gene or to an allelomorph thereof.

At the same time the following crosses were made. Shortbristle flies were inbred in three cultures (3443-44, 3453) in order to secure a homozygous stock, sb females were mated (3442) to males from the so-called III-ple stock — a stock which is homozygous for a series of Ill-chromosome recessives—a short-bristle female was crossed to a Star Dichaete male (3478) and conversely Star Dichaete females to sb males (3479). Of these cultures only the two latter gave offspring. The rest failed entirely to give any flies in spite of the fact that the P₁ individuals were living for more than ten days, and the culture medium showed no growth of mould which might be thought to have interfered with the growth of the larvae. Striking is the fact that the sb females were sterile in the three cultures in which they were crossed to sb males, while they gave offspring in two out of the three cultures in which they were crossed to unrelated males. This differential fertility could not be due to sterility of the sb males, since culture 3478 (SD ♀ ♀ × sb ♂ ♂) gave a large output of flies.

The sb character failed to reappear in F₁, thus demonstrating that the gene is recessive. In order to see to which linkage group the gene belonged an F₁ Star Dichaete male was crossed to a wild-type F₁ sister, heterozygous for the sb gene (3491, 7th May 1924). Star (S) is a dominant in the II, Dichaete (D) a dominant in the Ill-chromosome (at 40, 4). The test gave the following result : S D 66, S 45, D 118, wild-type 46; S D sb o, S sb 27, D sb o, sb 31. Thus, none of the Dichaete flies obtained showed the new character, while about a quarter of the Star flies did. Since there is no crossing over in the male, this result demonstrates that the new mutant gene is located in the Ill-chromosome.

The position of sb in relation to Dichaete was determined by aid of a female back-cross test carried out at the same time. A Star Dichaete F₁ female from culture 3478 was back-crossed to sb males (Table I.).

One of the F₁ Dichaete females from 3514 was also back-crossed to short-bristle males and gave (3554, 27th May 1924) D 53, sb 40; D sb 4,+ 7. When these data are included we get in a total of 598 individuals, 42 recombinations for D and sb, which corresponds to a recombination per cent, of 7.0 for these genes.

This indicates that the new recessive is located about 7 units to the right or to the left of Dichaete, thus confirming the result of the earlier test that the new gene is not allelomorphic to spineless which is located at 58.5, or 18.1 units to the right of Dichaete. Whether the sb locus is to the right or to the left of Dichaete was determined by aid of a back-cross test in which the Ill-chromosome dominants Dichaete and Hairless were both present (Table II.).

The Hairless gene (H) is located at 69.5. From the classes obtained in the test Table II, it is apparent that the sb gene lies to the right of Dichaete. In a total of 384 flies 13 or 3.4 per cent, are recombinations for D and sb, while 61, or 15.9 per cent., are recombinations for sb and H. This gives a recombination per cent, of 19.3 for D and H, while the map distance between these loci is 29.1. The standard recombination per cent, for these loci worked out by Bridges is 25.7, so the value obtained is somewhat lower than the expectation, particularly when the undetected double cross-overs within the sb—H distance are taken into consideration. The D—sb value is also lower than the one obtained in the first back-cross tests.

When the D—sb data of Table II. are added to those presented above, we get 55 recombinations for D and sb in a total of 982 flies. This corresponds to 5.6 per cent, of crossing-over between these two loci. The locus is accordingly at 40.4 + 5.6, i.e. at about 46 in the III-chromosome, or probably about midway between scarlet (43.8) and pink (48.0).

It will be noticed from the totals presented above that the viability of the sb mutants is lowered. Summing up we obtained in all 485 sb as against 646 non-sb flies, where equality is expected. The other mutants involved in the crosses are as to viability practically equal to the wild-type.

Another still more striking peculiarity of the short-bristle mutants is not apparent from the totals given, viz., their considerably delayed emergence. It was a general feature met with in all the cultures that during the first day’s counts only non-sb individuals occurred among the offspring. Shortbristle flies did not appear until the third or fourth day. The relative number of sb flies then rapidly increased, so that this class during the latter part of the 10-day counts formed the majority among the offspring. The single-day counts of culture 3489 (Table III.) may in this respect serve as an illustration.

It has been mentioned that the first attempts to procure a homozygous sb stock failed. The attempt was later repeated with mass cultures over and over again with negative result. The sb flies were occasionally seen to copulate, but no offspring were produced, and it was for a while believed that the short-bristle mutants were absolutely sterile inter se. Finally, however, two large mass cultures gave when about twenty days old a very limited numbers of sb offspring.

It is possible that this infertility is only apparent. Under ordinary conditions the larvae during the first days feed on the mould, thus preventing a harmful propagation of the latter. It might be thought that the mould gained the upper hand in the pure sb culture bottles due to the retarded development of the sb larvae, and that the unfavourable culture conditions thus created caused the death of the eggs and larvae. This explanation seems, however, not to hold true. It is repeatedly emphasised in the records that mass cultures of sb flies failed to contain any larvae or to give any offspring, also in culture bottles which after twenty days were practically free from mould. Since the attempt to keep a homozygous sb stock failed, the stock is kept by mating in each generation sb females to Dichaete males which are heterozygous for the sb gene. This stock cannot be made up at the same time intervals as the other stock cultures, since the necessary number of sb flies have not emerged when the other selected stocks have to be transferred into new culture bottles.

The lowered fertility of the sb flies is remarkable in the following respect. The sb males are of normal fertility. But it was exceedingly difficult to obtain any offspring when sb flies were mated inter se, in spite of the fact that the sb females were of fairly good fertility and productivity when crossed to wildtype males or to males from unrelated stocks. A certain lowering of the fertility and productivity was also apparent in the latter crosses, but this lowering was not marked enough so as to seriously interfere with the usefulness of the sb flies for ordinary experimental work.

With respect to this differential fertility the short-bristle mutant agrees with some other recessive mutants studied by Miss Lynch (1919). This author demonstrated that the sex-linked gene “fused’’and the II-chromosome recessive “morula” caused complete sterility of the homozygous females in matings with males of their own races. Females homozygous for the sex-linked recessive “rudimentary” were partially sterile when mated with males of their own race. When crossed to males of other races a certain percentage of fused, morula or rudimentary females gave a very limited amount of offspring.

Summing up, we find that the III-chromosome recessive short-bristle exhibits to a very striking degree the series of somatic and physiological peculiarities typical of the dominant Minute mutations. The only constant somatic effect of the sb gene is the reduced size of the bristles. The bristles have the same positions and directions as those of the wild fly and none are missing. The eyes which frequently seem to be slightly enlarged, show a tendency to roughening. This tendency is not apparent in the homozygous sb individuals. But it was observed in all the experiments involving Star that this gene which itself causes an irregular distribution of the ommatidia, was markedly enhanced in its effect, when present in flies homozygous for the sb gene. Just as in the Minutes there is a tendency for the sb flies to be paler in general colour, to show a dark trident pattern, to be smaller in size and to have blunter wings, which in some individuals show slight irregularities of the venation. There is also a tendency to irregularities of the abdominal bands, especially in the females.

This series of somatic Minute-characteristics is associated with considerably delayed emergence, lowered viability, and lowered female fertility. This lowering of the fertility is not very marked when sb females are mated with unrelated males, but when mated with males of their own race the sb females are entirely sterile in the large majority of the cases. Thus, whether the genotypical change is haploidy for an entire chromosome (as in Diminished), a dominant gene with homozygous lethal effect (as in the dominant Minutes), or an ordinary recessive gene (as in the mutant here recorded), the aggregate of phenotypical alterations is essentially the same. This situation clearly points in the direction that these different genetic changes must have a primary effect on some common developmental process, on which these different characters are dependent.

Sturtevant, who emphasises this in a short article on orthogenesis (1924), makes this group of mutant characters the object of some interesting considerations: The occurrence of “directive” evolution in characters that cannot be supposed to be of selective value is often held to be incompatible with the view that evolution results from the action of natural selection on random variations. Granted that orthogenetic series really occur, there remains a simple method of accounting for them without making use of any other primary factors than random variation and natural selection. A single gene normally produces changes in several characters. The somatic variations studied by the geneticist are very frequently associated with physiological variations. The physiological variations are the ones most likely to be acted upon by natural selection. Such selection would be expected to bring about changes in the associated somatic characters that are themselves of little or no selective value. If there is a general correlation between the variations in two or more characters, one of which is subject to natural selection, orthogenesis is to be expected. “It seems likely that natural selection, by operating to eliminate sterile females or slow growing larvæ, has kept the bristles of the species large and the eyes smooth.”