Calvin B. Bridges

The mutant “proboscipedia” in Drosophila melanogaster — A case of hereditary homoösis

SummaryThe recessive mutant „proboscipedia“, found byBridges April 27, 1931, has its locus in the third chromosome at approximately 5.3 units to the right of Dichaete, or at 45.7.In proboscipedia the oral lobes are converted into a labium-like organ, which also resembles a pair of antenna-like or tarsus-like appendages. This change, as well as the modifications of the labrum, maxillae, maxillary palpi, and other structures give to the mouth parts of proboscipedia a resemblance to those of the biting type found in insects of lower orders.The four-fold resemblance of the modified oral lobes in proboscipedia to the labrum, the maxillary palpi, the antenae and the tarsi is comprehensible in view of the homology of these three head-appendages with each other and with the walking legs.ZusammenfassungDie rezessive Mutante „Proboscipedia“, die vonBridges am 27. 4. 31 gefunden wurde, ist im 3. Chromosom ungefähr 5,3 Einheiten rechts von „Dichaete“, oder bei 45,7 lokalisiert.Bei Proboscipedie sind die oralen Lappen zusammengebogen zu einem lippenartigen Organ, das einem Paar antennenähnlicher oder tarsusähnlicher Anhänge gleicht. Diese Abart und ebenso die Modifikationen des Labrum, der Maxillen, der Maxillargliedmaßen und anderer Strukturen geben den Mundteilen von Proboscipedia eine Ähnlichkeit mit denen des kauenden Typus, wie er bei Insekten niederer Ordnung gefunden wurde.Die vierfache Ähnlichkeit der modifizierten oralen Lappen bei Proboscipedia hinsichtlich des Labrum, der Maxillargliedmaßen, der Antennen und der Tarsi ist verständlich da diese drei Kopfanhänge und die Laufbeine homolog sind.

The developmental stages at which mutations occur in the germ tract

In the fruit-fly Drosophila melanogaster about 300 primary mutations have been found, most of which arose in cultures carried on in the laboratory. A study of the critical cases among these mutations has shown that a large majority of them originated at or very near the maturation stage; that a few occurred in the gonial cells some time prior to maturation; and that a few occurred early in the segmentation stage. The conclusion that most mutations occur at the maturation stage is based largely on the proportion of sex-linked recessives and of dominants that have been first found as a single individual. Approximately half of the sex-linked recessives have been discovered as a single male. This is a surprisingly large proportion and clearly means that in these cases the actual mutation occurred in the mother, and at, or not more than a very few cell generations before, the maturation of the egg. Those sex-linked recessives that did not first appear as a single male have in the main appeared as half the sons of a female already heterozygous for the gene. In these cases the actual mutation had occurred at some indeterminate stage one or more generations previous to the appearance of the character. There are now about 30 known dominants in Drosophila melanogaster, of which fully two-thirds were first found as single heterozygous individuals. This very large proportion of dominants appearing as single individuals means that the actual mutation has occurred very close to the final stage in the formation of the gamete—probably little if any prior to maturation. That mutations may occur in the oijgonial cells prior to maturation is proved by a few cases in which a single female has given rise to more than a single individual of a new sex-linked recessive character.

Recombination and Crossing-Over

or 70.4 per cent., are carcinomas and 29.6 per cent. are adenomas. The figures certainly show no decrease and possibly an increase in malignancy in the tumors of yellow animals. This evidence suggests further the improbability that a general metabolic factor is involved. With these facts in mind we may conclude that a careful breeding experiment, using agouti (A) rather than yellow mice (AY) as the low cancer strain, should be carried out. This should suffice to show whether linkage between high incidence of cancer of the breast and the non-agouti (a) loeus exists. Whether it does or not, a difference in incideence of mammary cancer between yellows and non-yellows is already clearly established and forms the first evidence of an important interrelationship between a color variety of mice and the spontaneous incidence of mammary cancer. C. C. LITTLE

The construction of chromosome maps

The accuracy with which a chromosome map may be constructed depends upon several conditions. (1) The mutant characters employed should be carefully restricted to those cleanly separable both from the wild type and from each other, and whose viability is practically the same as that of the wild type. (2) Mutants should be selected whose loci are properly spaced—not so close together that the error of random sampling is excessive, nor so far apart that double crossing over occurs between them. (3) When the amount of double crossing over between two distant loci is accurately known, data involving them can be used by making the appropriate correction. (4) The data must be obtained under uniform conditions, special attention being paid to the age of the parents, constancy and suitability of temperature, and to freedom from genetic modifiers of crossing over. (5) Any experiment involving more than two loci should figure only once in the calculation of each particular region of the chromosome. (6) Data for each region should be adequate in amount as judged by the laws of probability. (7) If slightly different positions are indicated by two or more independent experiments, then a mean position should be calculated in accordance with the amount and value of the different sets of data. (8) The framework of the map having been constructed on the basis of the most significant loci, each remaining locus is interpolated as accurately as the amount and reliability of data permit.