par Francis FETTER Coexistence au Sahara occidental de deux especes de lievres Lepus Crawshay i et Lepus capensis qui remet en question la systeraatique des lievres de 1'Ancien Monde. Trois especes principales vivent en Afrique : L. Crawshayi, L. saxatilis et L. capensis. Lepus capensis est probablement une < grande espece > dont L. europaeus n'est qu'une forme et dont la repartition couvre l'Afrique, l'Europe et 1'Asie. Brought to you by | University of California Authenticated Download Date | 6/3/15 12:35 AM RäVISION DES LlfeVRES A FRIG AI N S 43 a Beni-Abbes, ces lievres coexistent avec des representants de L. capensis ä email incisif simple (fig. 2, en bas). Ces faits remettent en question la systematique des lievres de FAncien Monde et permettent d'envisager favorablement Fhypothese selon laquelle L. capensis ne serait que Tune des formes (la forme type) d'une « grande espece », englobant notamment L. europaeus, L. tolcü et L. thibetanus, et dont la repartition d'ensemble couvrirait TAfrique, FEurope et PAsie. Les formes qui paraissent referables a cette espece sont : L. s. saxatilis F.

A study of sex chromosomes and synaptonemal complexes in male specimens of Gerbillus chiesmaní, G. nigeriae, G. hoogstrali, and Taterillus pygargus is reported. In each of these Gerbillidae species there are two or three translocations of autosomes with X and Y chromosomes. Analysis of mitotic chromosomes consistently shows the presence of constitutive heterochromatin on the der t(X;autosome) at the X-autosome junction and on the der t(Y;autosome). Analysis of the synaptonemal complexes shows the existence of an unusual structure, lightly stained, at the X-autosome junction and at the Y-autosome junction, which is probably heterochromatic in nature, thus corresponding to the mitotic patterns. This heterochromatin separates the autosomal and gonosomal segments, which behave independently and normally. By analogy with findings from humans and other mammals, a general hypothesis is proposed on the role of intercalated heterochromatin between translocated gonosomes and autosomes. This hypothesis explains why the pathological consequences of these translocations may be very different in males and females. The role of intercalated heterochromatin would be to avoid the pathological consequences of gonosome-autosome translocations resulting from inactivation of the sex chromosomes in female somatic cells and male germinal cells.

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The sex chromosomes of 11 species of Gerbillidae (Meriones tristrami, M. crassus, M. libycus, M. persicus, M. unguiculatus, Gerbillus hesperinus, G. nigeriae, G. gerbillus, G. campestris, Gerbillurus tytonis, and Taterillus gracilis) are reported. A very complex evolution of the X chromosome and, to a lesser degree, of the Y chromosome was observed, including several inversions, translocations with autosomes, and an increase in heterochromatin. The role of constitutive heterochromatin, isolating autosome and gonosome segments in translocations (thus preventing any position effect), is discussed. The sequence of rearrangements affecting the sex chromosomes is considered with a view toward establishing the phylogenetic relationship of the species studied. The tendency to accumulate the same rare type of rearrangement, namely, gonosome-autosome translocations, is another demonstration that chromosomal evolution is not random in a given group of species.