How Green Are Biofuels?

Y chromosome in a single fly strain (ensuring an otherwise common genetic background). They then screened a nearly genome-wide set of genes and looked for differences in gene expression in response to the substituted Y chromosome. Because of high statistical error rates when thousands of genes are scored simultaneously, it is difficult to estimate the exact number of affected genes, but the estimates are surprisingly high, ranging from around 100 up to about 1000 (D. melanogaster is estimated to have ~13,000 genes). Lemos et al. also detected several coherent patterns among the Y-regulated genes. The Y chromosome influences the expression of genes that are more strongly expressed in males. Genes regulated by the Y chromosome are also more strongly influenced by environmental stress (heat shock), and many are associated with sperm development. Genes that influence the mitochondria are also overrepre-sented in the pool of genes regulated by the Y chromosome. In addition, affected genes are more evolutionarily dynamic in terms of poly-morphisms for gene expression within the species, and more diverged from a closely related congener (Drosophila simulans). It is now well established that a large proportion of the genome is expressed at different levels in males and females in many organisms (3), and the patterns found by Lemos et al. fit well with what would be expected for Y-linked regulatory genes. Whereas the Y chromosome spends every generation in males, the X chromosome and autosomes alternate between the sexes across generations. A mutation favoring males that is located on the X chromosome or autosomes can therefore only accumulate when selection in females is con-cordant, absent, or not too strongly discordant (4). However, this restriction is removed for Y-linked mutations because there can be no counterselection in females. Even when a mutation results in a phenotype that is exclusive to males, it will have an advantage if Y-linked because, unlike the X chromosome and autosomes, the Y chromosome is expressed (and hence selected) in males every generation (see the figure). The Y chromosome therefore represents a favorable platform for mutations that improve male gene expression. As many Y-linked genes have degenerated, should we expect the Y chromosome to inevitably be lost altogether? Not necessarily. As the number of functional genes on the Y chromosome declines, the efficacy of natural selection increases on the remaining genes. Decay of the Y chromosome therefore slows down over time and can ultimately stop altogether. …