Arrangement of bristles as a function of bristle number on a leg segment inDrosophila melanogaster

Abstract

The arrangement of bristles on a leg segment of the fruitflyDrosophila melanogaster was studied in various mutants that have abnormal numbers of bristles on this segment. Eighteen mutations at six different genetic loci were analyzed, plus five double or triple mutant combinations. Recessive mutations at theachaete-scute locus were found to affect distinct groups of bristles:achaete mutations remove mechanosensory bristles, whereasscute mutations remove mainly chemosensory bristles. Mechanosensory bristles remain uniformly spaced along the longitudinal axis unless their number decreases below a certain threshold, suggesting that spacing is controlled by cell interactions that cannot function when bristle cells are too far apart. Above a certain threshold, bristle spacing and alignment both become irregular, perhaps due to excessive force from these same interactions. Chemosensory bristles occupy definite positions that are virtually unaffected by removal of individual bristles from the array. Extra chemosensory bristles develop only near the six normal sites. At two of the six sites the multiple bristles tend to exhibit uniform longitudinal spacing - a property confined to mechanosensory bristles in wild-type flies. To explain the various mutant phenotypes the following scheme is proposed, with different mutations directly or indirectly affecting each step: (1) spots and stripes are demarcated within the pattern area, (2) one bristle cell normally arises within each spot, multiple bristle cells within each stripe, (3) incipient bristle cells inhibit neighboring cells from becoming bristle cells, and (4) the bristle cells within each stripe become aligned to form rows and then repel one another to generate uniform spacing.

Evolution of allometric changes in fruit fly legs: a developmentally entrenched story

“…We hypothesize that a developmental mechanism, lateral inhibition, is responsible for the coupling between t1 length and TR number (Held, 1990;Orenic et al, 1993;Held, 2002). Regular spacing between bristle rows occurs because the future bristle cells, the sensory organ precursors, inhibit the surrounding epithelial cells from having a neural cell fate (Held, 1990;Orenic et al, 1993;Held, 2002).…”

“…Regular spacing between bristle rows occurs because the future bristle cells, the sensory organ precursors, inhibit the surrounding epithelial cells from having a neural cell fate (Held, 1990;Orenic et al, 1993;Held, 2002). Interestingly, although several genetic perturbations can modify the distance between bristles (Held, 1990;Held, 2002), our data show that this type of modification is not usually the path taken during evolution ( Fig. 5D-E).…”

Analysis of a species/instars/characters table: a theoretical survey on the use of chaetotaxy in ontophylogenetic studies

“…Several authors (e.g., Doe & Goodman, 1985;Campos-Ortega, 1988;Held, 1990a;Simpson, 1990;Heitzler & Simpson, 1991) showed that the segregation of neural and epidermal lineages in Drosophila melanogaster relies on cellular interactions that are embodied in the concept of "lateral inhibition". A cybernetic model was put forward in which chance played a part.…”

“…A related problem is the geometrical pattern in which setae are often arranged. In his survey of basitarsal setae in D. melanogaster, Held (1990a: 61) stated that "the development of the basitarsus appears to be governed by a hybrid mechanism involving both a global coordinate system and local cell interactions". The coordinate system controls the arrangement of setae in rows and/or in whorls, and is likely to govern the positions of idionymic setae (i.e., setae with fixed positions).…”

“…The effect of genes on the presence or number of setae has been studied in detail by Held (1990a) on the second-leg basitarsus of D. melanogaster.…”