1 Dipartimento di Biologia Animale e dell’Uomo, Universita ` di Roma ÔLa SapienzaÕ, Rome, Italy; 2 Biology Department, Addis Ababa University, Addis Ababa, Ethiopia; 3 Pest Management Centre, Sokoine University of Agriculture, Morogoro, Tanzania; 4 Department of Zoology, Kenyatta University, Nairobi, Kenya Chromosomal diversity in the genus Arvicanthis (Rodentia, Muridae) from East Africa: a taxonomic and phylogenetic evaluation R. Castiglia 1 , A. Bekele 2 , R. Makundi 3 , N. Oguge 4 and M. Corti 1 Abstract In this paper we discuss the contribution of cytogenetics to the systematics of Arvicanthis in East Africa, by reviewing all the known chromosomal cytotypes of the genus in the area. We also provide G- and C-banding comparisons for two recently described karyotypes, provisionally named ANI-5 (2n ¼ 56, NFa ¼ 62) and ANI-6 (2n ¼ 60, NFa ¼ 72). This, therefore, brings the total number of known cytotypes in this area to 10. Five of these correspond to the species recognized by the latest rodent checklist, i.e. A. nairobae (2n ¼ 62, NFa ¼ 78), A. neumanni (2n ¼ 52–53, NFa ¼ 62), A. blicki (2n ¼ 48, NFa ¼ 62), A. abyssinicus (2n ¼ 62, NFa ¼ 64) and A. niloticus (2n ¼ 62, NFa ¼ 60–62). The taxonomic status of the remaining five cytotypes (A. cf. somalicus,2n ¼ 62 NFa ¼ 62–63; ANI-5, 2n ¼ 56, NFa ¼ 62; ANI-6/6a 2n ¼ 60, NFa ¼ 72/76; ANI-7, 2n ¼ 56, NFa ¼ 78; and ANI-8, 2n ¼ 44, NF ¼ 72) is discussed. Finally, we reconstruct the phylogenetic relationships among all the known karyotypes on the basis of banding data available for the genus in Africa and show the occurrence of two main clades, each characterized by different types of chromosomal rearrangements. The times of the cladogenetic events, inferred by a molecular clock, indicate that karyotype evolution has accomplished almost all the dichotomic events from the end of the Miocene to the present day. The discovery of a large chromosomal differentiation between populations showing low genetic distances and intrapopulation chromosomal polymorphism suggests that the process of chromosomal differentiation in Arvicanthis is still ongoing and may possibly be responsible for speciation. Key words: Arvicanthis – cytogenetics – cytotaxonomy – karyotype evolution – chromosomal phylogeny A taxonomic overview of the genus Arvicanthis The correct evaluation of species distribution and their taxonomic status is a fundamental prerequisite in assessing biodiversity in any geographical region. This constitutes the main challenge prior to any conservation planning and/or biodiversity management. For some groups this objective is problematical due to the occurrence of cryptic species and therefore taxonomic uncertainty. In rodents this is common because of the high morphological uniformity compared to important genetic diversity (Taylor 2000; Corti 2001; Granjon and Dobigny 2003). The high frequency of cryptic species occurring in syntopy or in allopatry, makes the classical morphological approach inadequate to determine the entire rodent diversity in many geographical areas. Morphology on its own, therefore, cannot form the basis of future biodiversity studies. In the last 30 years, however, molecular and cyto- genetic techniques have greatly improved the taxonomic resolution for many genera (Corti 2001). These studies have shown that species recognition requires a multidisciplinary approach based on cytogenetics (see for example Taylor 2000), the estimate of genetic divergence and DNA taxonomy (Blaxter 2004) and morphometrics (Loy et al. 2004). The African Arvicanthine rats of genus Arvicanthis Lesson, 1842 represent an emblematic example of this kind of problem. Several species are widely distributed in all the sub-Saharan African savannahs, along the Nile valley and down to the Zambezi. Their opportunistic and generalized diet makes them very common in agricultural fields and particularly in staple crops where they cause serious pre- and post-harvest damage (Stenseth et al. 2003). The systematics of the genus was recently improved by molecular (Ducroz et al. 1998), cytogenetic (Capanna et al. 1996; Volobouev et al. 2002b; Castiglia et al. 2003) and geometric morphometric investigations (Fadda and Corti 2001), so that now there are seven recognized species in the most recent checklist (Musser and Carleton, 2006) versus the five which had previously been accepted by the same authors (Musser and Carleton 1993). This increase in species number depends on the studies carried out recently in West Africa. In fact, through cytogenetics and molecular genetics (Volobouev et al. 1987; Civitelli et al. 1995; Ducroz et al. 1997, 1998) four distinct species, chromosomally well differentiated, have been identi- fied which were previously included in the single species A. niloticus (Desmarest, 1822). These karyotypic variants have been reviewed by Volobouev et al. (2002b) and assigned to A. niloticus (2n ¼ 62; autosomal fundamental number, NFa ¼ 62–64; Niger, Chad, Mali, Senegal, Burkina Faso, Mauritania), A. rufinus (Temminck, 1853) (2n ¼ 62; NFa ¼ 74; Benin), A. ansorgei Thomas, 1910 (2n ¼ 62; NFa ¼ 74–76; Burkina Faso, Mali, Senegal). In addition, another unidentified species has been found in the Central African Republic, which has been provisionally named ANI-2 (2n ¼ 58; NFa ¼ 72). The taxonomic resolution for the East African species is, however, more complex. Musser and Carleton (2006) recog- nized the occurrence of five species in this region, namely A. nairobae J. A. Allen, 1909, A. neumanni (Matschie, 1894) (A. somalicus in Musser and Carleton 1993), A. blicki Frick, 1914, A. abyssinicus (Ru¨ ppell, 1842) and A. niloticus (Table 1). There are however uncertainties regarding the taxonomy and systematics of this last species in the region: in fact, Yalden et al. (1976, 1996) maintained that an additional species occurs in Ethiopia on the basis of morphological differences, i.e. A. dembeensis (Ru¨ppell, 1842), which in sub- sequent work was included in A. niloticus on the basis of chromosomal (Corti et al. 1996) and molecular homogeneity (Ducroz et al. 1998). Moreover, Orlov et al. (1992) and Ó 2006 The Authors Journal compilation Ó 2006 Blackwell Verlag, Berlin Accepted on 31 December 2005 JZS doi: 10.1111/j.1439-0469.2006.00356.x JZS (2006) 44(3), 223–235