Highly variable recessive lethal or nearly lethal mutation rates during germ-line development of male Drosophila melanogaster

Abstract

Each cell of higher organism adults is derived from a fertilized egg through a series of divisions, during which mutations can occur. Both the rate and timing of mutations can have profound impacts on both the individual and the population, because mutations that occur at early cell divisions will affect more tissues and are more likely to be transferred to the next generation. Using large-scale multigeneration screening experiments for recessive lethal or nearly lethal mutations of Drosophila melanogaster and recently developed statistical analysis, we show for male D. melanogaster that (i) mutation rates (for recessive lethal or nearly lethal) are highly variable during germ cell development; (ii) first cell cleavage has the highest mutation rate, which drops substantially in the second cleavage or the next few cleavages; (iii) the intermediate stages, after a few cleavages to right before spermatogenesis, have at least an order of magnitude smaller mutation rate; and (iv) spermatogenesis also harbors a fairly high mutation rate. Because germ-line lineage shares some (early) cell divisions with somatic cell lineage, the first conclusion is readily extended to a somatic cell lineage. It is conceivable that the first conclusion is true for most (if not all) higher organisms, whereas the other three conclusions are widely applicable, although the extent may differ from species to species. Therefore, conclusions or analyses that are based on equal mutation rates during development should be taken with caution. Furthermore, the statistical approach developed can be adopted for studying other organisms, including the human germ-line or somatic mutational patterns.

Fig. 1.

Example genealogy of five alleles (a–e) sampled from the cell population after the fifth division. The value under a branch is the size of that branch (i.e., the number of descendant cells in the sample). Cell divisions are divided into three intervals, 1 [1, 1], 2 [2, 4], and 3 [5, 5], which lead to t^T = (1, 9, 5).

Fig. 2.

Illustration of the relationship of developmental stages, dynamics of population sizes, and genealogy of a sample of five spermatozoa. The last five cell divisions represent spermatogenesis, which may start as early as the 32nd division.