POLYMORPHISME CHROMOSOMIQUE (lEGGADAS) 



355 



SUMMARY 



This work rests on the cytogenetic analysis of 150 pigmy-mice (Leggada) 

 from the Sénégal, the Ivory Coast and the Central African Republic, among them 

 some individuals belonging to both new species recently described by Petter as 

 Mus oubanguii and Mus goundae. M. oubanguii {2N=2S) shows a polymorphismus 

 due to pericentric inversions with fundamental numbers ranging from 30 to 34. 

 By M. goundae (2N=16, 17, 18, 19) we have to deal with a Robertsonian poly- 

 morphism {F.N.^Z^). 



Sex-chromosomes — There are two main types: in the « primitive » type (PR) 

 the X and the Y are acrocentric, like those of M. musculus. In the « translocated » 

 type the X and the Y are meta- or submetacentric. Following the hypothesis of 

 Matthey (1966), this shape could resuit from the translocation of PR sex- 

 chromosomes on the mates of an autosomic pair. The autoradiographic method 

 gives results in full agreement with this hypothesis, every sex-chromosome 

 showing an euchromatic and an heterochromatic arm. Among the TR-m\CQ there 

 are many varieties to be considered. 



The cytological dates provide criteria which deserve to be introduced in a 

 revision of the taxonomy. The author gives a discussion on this topic and, more 

 generally, on the systematics of the genus Mus. To investigate the zoo-geo- 

 graphical problem set up by the distribution of the pigmy-mice, the author adds 

 to her sample of 150 animais the 357 individuals studied previously by Matthey 

 (1958-1970). 



The superspecies M. minutoides/musculoides appears particularly interesting: 

 in this species the following chromosome numbers were known, 18, 19, 22, 31, 

 32, 33, 34. The discovery of spécimens with 20, 21, 23, 24, 25, 26, 27, 28 29 and 30 

 chromosomes adds the missing rays to the « Robertsonian fan ». We take notice 

 that ail the différent caryotypes do not occur together in the same locality; eight 

 was the maximum observed. It seems rather obvious that every caryotype reacts 

 differently to the local pressure of sélection. 



The author expounds the following hypothesis: the numerous chromosomal 

 mutations leading to the polymorphismus must be considered as closely related 

 with the instable state of the genetic System (Genetic révolution of Mayr) and as 

 indicative of this instability. According to this conception, the successive phases 

 of the speciation are depicted. 



Thèse facts are in good agreement with the hypothesis that the pigmy-mice 

 are scattered in small populations. In such isolâtes, the conditions favour the 

 building up of new species. It is probable that in some cases we have to deal with a 

 sympatric speciation. 



