Alfred Sturtevant and George Beadle Untangle Inversions.

Great articles often begin with an intriguing paradox, describe an elegant experimental approach, provide lasting and important data, and change the course of their discipline. Sturtevant and Beadle (1936) meets all of those criteria and stands as a paradigm for the genetic analysis of chromosome behavior in Drosophila. It began with two paradoxes, both ofwhichwere vexing but not obviously connected. First, females heterozygous for paracentric inversions, which do not include the centromere, failed to produce progeny bearing single crossovers within the inversion but did produce progeny bearing double crossovers. There was no change in the number of eggs hatched, ruling out inviability of eggs containing single crossover chromosomes as an explanation. Second, although such females only rarely produced progeny bearing two maternal X chromosomes, they frequently produced progeny with no maternal X chromosomes (patroclinous males). How was inversion heterozygosity producing such an odd set of meiotic anomalies? Sturtevant and Beadle demonstrated that single crossovers did indeedoccurwithin the inverted segmentsby characterizing crossing over in attached-X chromosomes, where both the normal X and its inversion-bearing homolog were attached to a single centromere. The arms of attached-X chromosomes (being homologs) undergo pairing and crossing over. These authors found that the occurrence of single crossovers within the inverted region in such chromosomes generated ring-X chromosomes at expected frequencies for the larger inversions and substantial frequencies for the smaller ones. These observations led Sturtevant and Beadle to conclude that in the inversion heterozygotes, “. . .single crossover chromatids are selectively eliminated during themeiotic process.” But how? And is that “selective elimination” tied to the production of those patroclinous exceptions? Sturtevant and Beadle proposed that themechanism for selective elimination lay in the fact that meiosis in Drosophila involves only nuclear division within the oocyte—no cell division occurs (Huettner 1924). The four meiotic nuclei are arranged in a row perpendicular to the egg cortex. Only the innermost nucleus participates in fertilization; the other three are eliminated. A single crossover within a paracentric inversion generates two (noncrossover) parental types and two recombinant products: one acentric fragment lacking a centromere and a complementary dicentric chromosome with a chromatin bridge connecting twohomologous centromeres. The acentric fragment cannot attach to the meiotic spindle and is lost, but what becomes of the dicentric chromosome and the two nonrecombinant chromatids that compose themeiotic tetrad? Sturtevant and Beadle proposed that