SR chromosomes are the best-known case of sex chromosome meiotic drive. These X chromosomes cause the production of female-biased progenies in several Drosophila species. Due to their meiotic drive advantage, they are expected to spread and become ®xed, resulting in population extinction due to the lack of males. However, this apparently does not occur: SR chromosomes are maintained in balanced polymorphisms, resulting from the equilibrium between their meiotic drive advantage and deleterious ®tness e ects. In this paper we review the current explanations for their deleterious e ects and we argue that it is highly improbable that all newly emerged SR are su ciently deleterious to avoid ®xation. Unbalanced SR almost certainly arise and go unnoticed because of three possible outcomes: (i) ®xation followed by extinction of the population or species; (ii) ®xation followed by the emergence and ®xation of drive suppressors, restoring the normal 1:1 sexual proportion; or (iii) transformation into balanced SR due to partial suppression. If these outcomes really occur, then extant cases of sex-chromosome meiotic drive such as SR, causing small deviations on the population sexual proportion, are only the tip of the iceberg and strong sexual proportion shifts (possibly followed by extinction) are a more common feature of species evolution than is usually assumed.

The sex-ratio trait is the production of female-biased progenies due to X-linked meiotic drive in males of several Drosophila species. The driving X chromosome (called SR) is not fixed due to at least two stabilizing factors: natural selection (favoring ST, the nondriving standard X) and drive suppression by either Y-linked or autosomal genes. The evolution of autosomal suppression is explained by Fisher's principle, a mechanism of natural selection that leads to equal proportion of males and females in a sexually reproducing population. In fact, sex-ratio expression is partially suppressed by autosomal genes in at least three Drosophila species. The population genetics of this system is not completely understood. In this article we develop a mathematical model for the evolution of autosomal suppressors of SR (sup alleles) and show that: (i) an autosomal suppressor cannot invade when SR is very deleterious in males (c Ͻ

Y chromosomes are widely believed to evolve from a normal autosome through a process of massive gene loss (with preservation of some male genes), shaped by sex-antagonistic selection and complemented by occasional gains of male-related genes. The net result of these processes is a male-specialized chromosome. This might be expected to be an irreversible process, but it was found in 2005 that the Drosophila pseudoobscura Y chromosome was incorporated into an autosome. Y chromosome incorporations have important consequences: a formerly male-restricted chromosome reverts to autosomal inheritance, and the species may shift from an XY/XX to X0/XX sex-chromosome system. In order to assess the frequency and causes of this phenomenon we searched for Y chromosome incorporations in 400 species from Drosophila and related genera. We found one additional large scale event of Y chromosome incorporation, affecting the whole montium subgroup (40 species in our sample); overall 13% of the sampled species (52/400) have Y incorporations. While previous data indicated that after the Y incorporation the ancestral Y disappeared as a free chromosome, the much larger data set analyzed here indicates that a copy of the Y survived as a free chromosome both in montium and pseudoobscura species, and that the current Y of the pseudoobscura lineage results from a fusion between this free Y and the neoY. The 400 species sample also showed that the previously suggested causal connection between X-autosome fusions and Y incorporations is, at best, weak: the new case of Y incorporation (montium) does not have X-autosome fusion, whereas nine independent cases of X-autosome fusions were not followed by Y incorporations. Y incorporation is an underappreciated mechanism affecting Y chromosome evolution; our results show that at least in Drosophila it plays a relevant role and highlight the need of similar studies in other groups.

In December 2006, 495 drosophilids belonging to 19 species were aspirated from inflorescences of Calathea monophylla (Vellozo) Körn at a forest reserve in the city of São Paulo, state of São Paulo, and 42 specimens belonging to three species of Drosophila were aspirated from those of Calathea cylindrica (Roscoe) Karl Schumann in an urban forest in the city of Rio de Janeiro, state of Rio de Janeiro, Brazil. In addition, 20 inflorescences of C. monophylla and 14 inflorescences of C.cylindrica were collected and observed for the emergence of flies in the laboratories. In total, 137 drosophilids belonging to Zygothrica dispar (Wiedemann, 1830) plus five species of Drosophila (Drosophila griseolineata Duda and four undescribed species) emerged from C. monophylla inflorescences, and a total of 22 specimens, all belonging to two undescribed species of Drosophila, emerged from those of C. cylindrica. Drosophila calatheae sp. nov., ungrouped but related to both the xanthopallescens Pipkin and the bromeliae Patterson & Stone species groups, is described based on both aspirated and emerged flies from C. monophylla inflorescences from São Paulo city. This new species was also reared in the laboratory with a new medium recipe, thus providing larvae for chromosomal studies.