Sex chromosomes can differ between species as a result of evolutionary turnover, a process that can be driven by evolution of the sex determination pathway. Canonical models of sex chromosome turnover hypothesize that a new master sex determining gene causes an autosome to become a sex chromosome or an XY chromosome pair to switch to a ZW pair (or vice versa). Here, a novel paradigm for the evolution of sex determination and sex chromosomes is presented, in which there is an (...) evolutionary transition in the master sex determiner, but the X chromosome remains unchanged. There are three documented examples of the novel paradigm, and it is hypothesized that a similar process could happen in a ZW sex chromosome system. Three other taxa are also identified where the novel paradigm may have occurred, and how it could be distinguished from canonical trajectories in these and additional taxa is also described. (shrink)

Graphical AbstractSquamates may be an attractive group in which to study the influence of sex chromosomes on speciation rates because of the repeated evolution of heterogamety (both XY and ZW), as well as an apparently large number of taxa with environmental sex-determination.

Die Kritische Sozialtheorie sowie die kommunistische Herrschaftsphilosophie haben in der posthegelianischen Befreiungslehre ihren Ursprung. Die Kritische Theorie versuchte diesen Denkansatz gegen seine totalitären Konsequenzen anzuwenden. Dieselben Weltdeutungschemata, die man an der Sowjetideologie anstößig fand, galten aber als akzeptabel, sobald man nur zur Kritik der westlichen Industriegesellschaft überging. Der Hauptpunkt der neomarxistischen Kritik des Sowjetsystems bestand darin, daß man die besondere institutionelle Form, in der der reale Sozialismus das Heil zu verwirklichen beanspruchte, in Frage stellte, ohne andere Lösungen der institutionellen Probleme (...) anzubieten oder das zugrundeliegende Denkprogramm aufzugeben, in dessen Kontext diese Probleme durchaus legitim gestellt werden konnten. Die relativen Leistungen der Kritischen Theorie sind in ihren sozial-politischen Folgen zu suchen, u. zw. darin, daß sie zwei Grundlagen der totalitären Ideologie, von der die Legitimierung des Gesellschaftssystems herrühte, erschütterte: (1) Verfügung über die Tradition; (2) Verfügung über die Sprache, in der über heilsrelevante Fragen gesprochen wurde. /// Both critical theory and communist philosophy arise from the posthegelian theory of salvation. Critical theory undertook the effort to preserve the hard core of this theoretical background and at the same time to criticize by its means the totalitarian consequences the communist ideology had derived from it. In effect, the very same patterns of thought which critical theory believed to denounce as instruments of totalitarian manipulation were accepted as soon as they proved useful to contest advanced industrial society and its world view. The crucial point of the new-leftist criticism of the Stalinist ideology was the rejection of a central claim of the latter according to which the salvation predicted in the leftist-revolutionary tradition has found its actual fulfilment in the institutional framework of the Soviet state. Challenging this claim, the Neomarxists neither abandoned this tradition nor offered another solution to the institutional problems which result from it. The relative achievements of critical theory lie in the damage it has caused to the two foundations of the totalitarian ideology, i.e.: (1) its authority over tradition and (2) its authority over language. (shrink)

Mammal sex determination depends on an XY chromosome system, a gene for testis development and a means of activating the X chromosome. The duckbill platypus challenges these dogmas.1,2 Gutzner et al.1 find no recognizable SRY sequence and question whether the mammalian X was even the original sex chromosome in the platypus. Instead they suggest that the original platypus sex chromosomes were derived from the ZW chromosome system of birds and reptiles. Unraveling the puzzles of sex determination and dosage compensation in (...) the platypus has been complicated by the fact that it has a surplus of sex chromosomes. Rather than a single X and Y chromosome, the male platypus has five Xs and five Ys. BioEssays 27:681–684, 2005. © 2005 Wiley Periodicals, Inc. (shrink)

Not all vertebrates share the familiar system of XX:XY sex determination seen in mammals. In the chicken and other birds, sex is determined by a ZZ:ZW sex chromosome system. Gonadal development in the chicken has provided insights into the molecular genetics of vertebrate sex determination and how it has evolved. Such comparative studies show that vertebrate sex‐determining pathways comprise both conserved and divergent elements. The chicken embryo resembles lower vertebrates in that estrogens play a central role in gonadal sex differentiation. (...) However, several genes shown to be critical for mammalian sex determination are also expressed in the chicken, but their expression patterns differ, indicating functional plasticity. While the genetic trigger for sex determination in birds remains unknown, some promising candidate genes have recently emerged. The Z‐linked gene, DMRT1, supports the Z‐dosage model of avian sex determination. Two novel W‐linked genes, ASW and FET1, represent candidate female determinants. BioEssays 26:120–132, 2004. © 2004 Wiley Periodicals, Inc. (shrink)

The text quoted above each note is that of the edition of Seneca's tragedies by Otto Zwierlein , OCT 1986; numerous passages are discussed in his Kritischer Kommentar zu den Tragüdien Senecas , Stuttgart, 1986; various textual suggestions were made in a correspondence with Zw. by B. Axelson . Other works on Seneca's tragedies, referred to by the scholar's name only, are: Text and translation: F. J. Miller, Loeb, 1917; L. Herrmann, Budé, 1924–6. Text with commentary: R. J. Tarrant, Agamemnon (...) , and Thyestes ; J. G. Fitch, Hercules Furens . Text with commentary and translation: Elaine Fantham, Troades ; A. J.Boyle, Phaedra. (shrink)