Rend. Fis. Acc, l.ancei s. 9, v. 6:157-164 (1995)

Genetica. - - The chromosomes of three endemic rodents o f the Bale mountains,

South Ethiopia. Nota di MARco CoRal, MAmA VrrroRiA ClVnXLLI, AFZWOV, I( BEmSLE, RlCCAm)O CASTIGLIA e EV, rqESTO CAI'ANNA, presentata(*) dal Corrisp. E. Capanna.

ABSTRACT. - - Karyotypes of three endemic Rodents of the Bale Mountains (South Ethiopia) are pre- sented. These are Arvicantbis blicki, Lophuromys melanonyx, and Stenocephalemys albocaudata, for which no karyological information was previously available. These new data are discussed within the context of chro- mosomal rearrangements and microevolutionary events.

KEY WORDS; Chromosomes; Rodents; Cytotaxonomy; Ethiopia.

Rmssur, rro. - - I cromosomi di tre specie di Roditori endemici del massiccio del Bale, Sud Etiopia. Sono presentad i cariotipi di tre specie di roditori, Arvicanthis blicki, Lophuromys melanonyx, e Stenocephalemys al- bocaudata, per i quali non era ancora noto il dato cariologico, endemici del complesso montuoso del Bale (Sud Etiopia). I dad vengono discussi neU'ambito dei problemi cormessi al ruolo del riordinamento stmt- turale del cariodpo nei processi microevolutivL

INTRODUCTION

The Bale Mountains National Park is part of the Southeast Ethiopian highlands and extends between 07 ~ 0 0 ' N and 39 ~ 45' E. It covers an area of 2471 km 2 . The altitude ranges from 1500 to 4377 m asl with the mean annual rainfall of 1150 mm. The park cov- ers a very extensive Afro-alpine area above 3700 m asl (1000 km 2 ). This encompasses the largest area of Afro-alpine habitat on the African continent. The vegetation type includes Afro-alpine, high mountain, montane evergreen forest, montane savanna, montane ever- green thicket and scrub. Besides its natural beauty and landscape, it is an indispensable montane area in view of the number of endemic species it conserves.

The first description of the fauna and the flora of the Bale massif was carried out by Smeds (1959) and Moony (1963), and more recently by Miehe and Miehe (1994). The vegetation consists of Juniperus procera and Hagenia abyssinica to the North, Erica

species and Lobelia rynchopetalum on the moorland, and Podocarpus gracilior and Aningeria species in the South.

Due to the varieties of habitats, faunal diversity is high. Although the park is known for the presence of the endemics Nyala and Simien Jackal, it is also unique in the pres- ence of small mammals. Observation and description of new species of rodents in the Bale Mountains National Park were carried out by Dorst (1972), Petter (1972), Di- eterlen and Rupp (1978), Rupp (1980), Yalden (1988), Yalden and Largen (1992), Afe- work Bekele (1988) and Hillman (1986, 1993). Out of the 66 mammals recorded in the park, 17 (25.8%) are endemic. When it comes to rodents, the park harbours 18 species of rodents of which 50% are endemics.

Although the park is unique in its faunal composition and water catchment areas, the encroachment by humans is threatening the diverse habitats. It is therefore high

(*) Nella seduta deU'11 marzo 1995.

158 ~. CORal ~x 2u~.

time to carry out further research in this underexplored region. In the course of a field expedition during 1994 sponsored by the Accademia Nazionale dei Lincei, three species of endemic rodents have been collected, namely: Arvicanthis blicki, Frick 1914; Lophuromys melanonyx, Petter 1972; and Stenocephalemys albocaudata, Frick 1914; for which there was no karyological information available. They are all species occurring at high altitudes in the open moorland. The three species are syntopic in the Afro-alpine habitat but occur in different niches Arvicanthis blicki is a diurnal member of the moor- land fauna, above 3500 m asl, although it descends to grassland at lower altitudes; Lo- phuromys melanonyx shares its habitat with the former but it is considerably smaller; Stenocephalemys albocaudata is nocturnal and has a larger size and occurs above 3700 m asl and is characteristic of the Alchemilla scrub (Yalden et al., 1976).

In the present study we present a preliminary report of the cytogenetic of these species.

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THE CHROMOSOMES OF THREE ENDEMIC RODENTS ... 159

M A T E R I A L S A N D M E T H O D S

Animals were collected by use of Sherman traps in the moorland of the Sanetti plateau, 4000 m asl, in Bale Mountains National Park (fig. 1). The animals for which chromosome preparations were available are as follows: Arvicanthis blicki, one female; Lophuromys melanonyx, one male and two females; Stenocephalemys albocaudata, one m a l e .

Chromosome metaphases were obtained from bone marrow according to the method of Hsu and Patton (1969). Metaphases have been prepared at the Biology De- partment, Addis Ababa University, and were then stained by standard method Giemsa pH7 and analysed at the Dipa~mento di Biologia Animale e dell'Uomo, UniversitA di Roma <<La Sapienza>>. G-, C-, and Ag-NOR bands were also obtained.

Fig. 2. - Karyotype of Arvicanthis blicki. Female.

160 M. CORTI ET AL.

RESULTS AND DISCUSSION

Arvicanthis blicki (fig. 2).

The diploid number of this species is 2n = 48. The karyotype comprises 9 pairs of biarmed (metacentrics or submetacentrics) autosomes (nos. 1-8 of the karyotype); one of these pairs is very small (no. 9). Fourtheen acrocentrics decreasing in size complete the autosomal set. The autosomal fundamental number (aFN) is 64. Two submetacen- tric X chromosomes are identified by comparison with other species of the genus. Sub- metacentric chromosomes of the same size and shape occur in A. niloticus (Volobouev et al., 1987; Civitelli et al., 1995; Granjon et al., 1992), in A. abyssinicus (Orlov et al., 1989) and A. dembeensis (Orlov et al., 1989).

The astonishing evidence emerging from the comparison of the karyotype of A. blicki with the known karyotypes of the other species is its low diploid number. Indeed, in A. niloticus the wide variability of diploid number ranges from 2n = 56 to 2n = 62 (for a review see CiviteUi et al., 1995). A. dembeensis is characterized by an equal range of variability (Orlov et al., 1989). A. abissynicus shows a diploid number of 2n = 62 (Ortov et al., 1989). This decreasing in the diploid number is consequent to centric fu- sions which produced the high number of biarmed chromosomes typical of the kary- otype of this species. The precise assessment of the fusion pattern characterizing kary- otype evolution within the genus Arvicanthis is in progress throughout comparisons of G-, C-, and AgNOR-bands.

Concerning the decrease of diploid number in Arvicanthis, it is worth recalling the amazing karyotype with 44 chromosomes, mainly metacentrics (FN = 68), that Capan- na and Civitelli (1988) described for a Somalian specimen. That specimen was provi- sionally attributed to A. niloticus, but it should be likely considered a cryptyc new species.

Both Robertsonian mechanisms and pericentric inversions are the responsible of the evolution of the karyotype of the genus.

Lophuromys melanonyx (fig. 3).

The diploid number of the species is 2n = 60. The autosomal set includes 7 pairs of large metacentrics (nos. 1-7), two pairs of large submetacentrics (nos. 8, 9), five pairs of small metacentrics (nos. 10-14), and 15 pairs of acrocentric chromosomes decreas- ing in size. Accordingly, the aFN is 86. The sex complement shows a large submetacen- tric X chromosome and a medium size acrocentric Y chromosome.

The karyology of the genus is complex as a consequence of wide variability between and within species. For example, Lophuromys flavopunctatus from different localities shows different diploid numbers: 2n = 70 in Burundi and Zaire (Dieterlen, 1978; Maddalena et al., 1989) and 2n = 68 in Ethiopia (Orlov et al., 1989). A constant high diploid number is characteristic of all studied species of the genus: from 2n = 70 in L. flavopunctatus (Matthey, 1967) to 2n = 56 in L. nudicaudus

T H E C H R O M O S O M E S O F T H R E E E N D E M I C R O D E N T S .. . 161

Fig . 3. - K a r y o t y p e o f Lophuromys melanonyx. M a l e .

(Verheyen and van der Straeten, 1980). Also the species are characterized by a high number of biarmed chromosomes.

Stenocephalemys albocaudata (fig. 4).

The diploid number of S. albocaudata is 2n = 54. The autosomal set is composed of all acrocentric chromosomes, decreasing in size except the smallest pair which is formed by small metacentrics. The aFN is 54. The sex complement is constituted by a large metacentric chromosome and by a smaller metacentric Y chromosome.

There is no available information on the karyology of the single other species of the genus, i.e. S. griseicauda, which is currently under investigation by us. However, it might be of interest to compare with the karyotype of P. albipes. Yalden et al. (1976) indeed suggested that the genus Stenocepbalemys <<is closely related to Praomys, especially P. a[bipes>>. Their hypothesis is apparently in accordance with our unpublished findings on the karyology of P. albipes from Ethiopia. The karyotype of the latter (in which there occurs a consistent retained polymorphism) is characterized by a diploid number of 2n = 46, but the Fundamental Number varyes from 50 to 54 as in S. albocaudata. Moreover, the karyotype of P. albipes

162 M. CORT~ ET AL.

Fig. 4. - Karyotype of Stenocephalemys albocaudata. Male.

shows the same pair of small metacentrics and a very similar morphology of the sex pair of S. albocaudata.

CONCLUSIONS

The analysis of the karyotypes of these species occurring in the highest areas of the Ethiopian highland shows a distinct and relevant diversification from congeneric species, from which they have speciated in this Afro-alpine habitat. This contingency proposes to the attention the debated question whether chromosal rearrangements are a primary cause of speciation or rather a secondary effect of microevolutionary events following geographic isolation (Capanna and Redi, 1994). The peculiar environment of the Bale massif requires specific adaptation and population divergence; the causes of speciation seem rather allopatric and chromosomal rearrangements would constitute, in this case, a secondary product of isolation.

Nonetheless, chromosomal rearrangements may help in maintaining reproductive isolation in parapatric contacts as for L. melanonyx and L. flavopunctatus. Indeed, the former has a 2n = 60 karyotype and the latter has 2n = 70 (Orlov et al., 1989).

Finally, we would stress the need to study endemism such as those in the Bale mountains and in other Afro-alpine habitats as they may provide very important insights on the mechanisms of species differentiation.

THE CHROMOSOMES OF THREE ENDEMIC RODENTS ... 16~

ACIC,,rOWLEDGEMENTS

The authors are deeply indebted to Ato Goitom Redda (Biology Department, Addis Ababa), and to Wolde Gebrekidan (Bale mts National Park, EWCO), for their valuable help during 1994 field work, and to Dr. Maria Luisa Puccetti Azzaroli (Museo Zoologico della Specola, Firenze), who shared the costs and the labours of the 1994 Lincei's mission in Ethiopia. The authors also acknowledge the founding of Ac- cademia Nazionale dei Lincei (Commissione Musei Naturalistici, M. L. Puccetti Azzaroli and E. Capanna), of University of Rome <<La Sapienza>> (project <<Model of Evolution in Tropical Environments~), and C.N.R. (no. 93.4294.04).

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M. Corti, M. V. Civitelli, R. Castiglia, E. Capanna: Dipartimento di Biologia Animale e deU'Uomo Universit~t degli Studi di Roma ~La Sapienza~

Via Borelli, 50 - 00161 ROMA

Afework Bekele: Biology Department, Addis Ababa University

P.O. Box 1176, ADDES ABABA (Ethiopia)