Section 10- — Developmental Genetics 



identified alleles. They are: lethal yellow A y , 

 viable yellow A v ?, light bellied agouti A w , 

 agouti (wild type) A, intermediate agouti A\ 

 tanoid a ia , black and tan a 1 , non-agouti a, an ex- 

 treme non-agouti, a e . Many remutations from non 

 agouti a to black and tan a 1 , and to light bellied 

 agouti A w , have occurred. This report will sum- 

 marize the findings about these remutations; de- 

 scribe two of the newer alleles, viable yellow A vy 

 and intermediate agouti A 1 ; report some of the 

 information obtained about the action of these 

 alleles on different backgrounds; the appearance 

 and identification of various allelic combinations, 

 and review briefly some of the biochemical 

 studies and attempts to produce phenocopies of 

 certain alleles. 



Certain closely linked genes can modify the 

 expression of some members of this allelic series. 

 The results of crosses of members of the agouti 

 series with these modifying genes may contribute 

 to the understanding of the action of the agouti 

 locus and the interactions of the various members 

 of the agouti series. 



The action of the alleles of this series may 

 affect pigmentation of the entire coat or only 

 portions of the coat. Are these true alleles or 

 pseudo-alleles at a complex locus? This review 

 is being presented to open new avenues of 

 discussion and research to try to answer this 

 question. 



10.16. The Control of Bristle Pattern by Hairy- Wing 

 in Drosophila melanogaster. Frederick Jay 

 Gottlieb (Pittsburgh, U.S.A.). 



An analysis of the control of developmental 

 patterns exercised by the mutant gene Hairy-wing 

 49c {Hw i9c ) was undertaken. Inhomozygotesthe 

 thorax is disproportionately wider at the level of 

 the dorsocentrals, there are numerous extra 

 macrochaetae and an increase in the number of 

 achrostichal rows and chaetal density is noted. 

 The heterozygote has normal width, fewer 

 extra macrochaetae and extra achrostichal rows, 

 and no significant increase in chaetal density. 



A developmental analysis of the differences 

 in differentiation of Hw iSc /Hw^, Hw 49c /Hw+, 

 and Hw+/Hw+ tissues was performed by means 

 of genetic mosaics, resulting from X-ray induced 

 somatic crossing-over in heterozygous larvae. 

 Measurements and counts in the dorsocentral 

 region were used. 



X-irradiation causes heterozygotes to express 

 themselves in a homozygous-like fashion. 



Interactions were observed between tissues of 

 the three genotypes in mosaics. Hw i9c /Hw 49c 

 tissue acts semi-autonomously in mosaics. 

 Hw + /Hw + tissue acts non-autonomously, oc- 



casionally producing extra macrochaetae in the 

 presence of adjacent Hw i9c /Hw 49c and, more 

 rarely Hw 49c /Hw + tissue. 



The evidence suggests that Hw 49c adds extra 

 width to the thorax on which new prepattern 

 properties can be exhibited. The rest of the 

 thoracic prepattern does not seem to be altered 

 by this mutant, but rather the competence of the 

 hypodermal tissue to respond to these areas of 

 the prepattern appears to be changed. 



This work was supported by a Predoctoral 

 Traineeship in Genetics, National Institutes 

 of Health, and is part of a thesis submitted in 

 partial fulfilment of the requirements for the 

 degree of Doctor of Philosophy in Genetics at 

 the University of California, Berkeley. 



10.17. The Developmental Control of Activity and 

 Expression of the Hairless Locus in Drosophila 

 melanogaster. David Nash (Cambridge, 

 Great Britain). 



The dominant mutant Hairless (H) on chromo- 

 some III of Drosophila melanogaster commonly 

 affects the macrochaetae of the head. Absence 

 of macrochaetae is not accompanied by severe 

 derangement of neighbouring pnaffected bristles. 

 Each vacant site may or may not bear a vestigial 

 chitinized disc. Selection for changes in level of 

 mutant expression indicates that one main rule 

 governs the distribution of vestiges and is best 

 interpreted as follows; 



(1) In normal development the chaetae at 

 different sites regularly develop at different 

 times. 



(2) Within development of a chaeta, later and 

 earlier deficiency of a normal gene product 

 result in presence and absence of vestiges 

 respectively. 



(3) By selection the form of the time-course of 

 deficiency of the normal gene product is changed. 



(4) Production of the hypothetical substance 

 is under negative feed-back control, which can- 

 not correct for the deficiency caused by the 

 mutant, but is still sufficient to produce a rise of 

 concentration after a critical level of deficiency 

 has been reached. 



Two specific modifiers of hairless exist. The 

 time-course of their effects can be deduced from 

 similar morphological considerations as above: 

 Suppressor- Hairless (Su-H) acts as if it sup- 

 presses Hairless more strongly as development 

 progresses. Enhancer Hairless (E-H) flies are 

 often nearly devoid of chaetae, the majority of 

 sites bearing vestiges. Enhancement of mutant 



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