Genetica 79: 93-105, 1989. © 1989 KIuwer Academic Publishers. Printed in Bel~um. 93 Inter- and intra-individual chromosome variability in Thamnomys (Grammomys) gazellae (Rodentia, Muridae) B-chromosomes and structural heteromorphisms M. V. Civitelli, P. Consentino & E. Capanna Department of Animal and Human Biology, University of Rome 'La Sap&nza', Rome, Italy Reccived 18.5.1988 Accepted in revised form 23.3.1989 Abstract The present paper reports intra- and inter-individual variability related to the occurrence of numerous B-chromosomes in Thamnomys (Grammomys) gazellae, a species of African Climber rat belonging to the "dolichums' group. The frequency of B-chromosomes in somatic and spermatogonial metaphases is investigated, together with their behaviour during meiosis. Moreover, G-banding makes it possible to identify a structural polymorphism resulting from a pericentric inversion in a large chromosome (no. 6). The distribution of the constitutive heterochromatin has been assessed by C-banding. The nucleolus organizer regions (NOR's) were located by means of silver staining in four chromosomal pairs (nos. 1, 2, 4, and 6). The karyotype of T. (G.)gazellae is compared with that of other taxa of the dolichurus group, particularly the Somaliland population which also exhibits the occurrence of B-chromosomes. The origin and significance of B-chromosomes is discussed. Introduction Chromosomal variability occurs frequently in rodents and has been related to the prolific speciation they display (Patton & Sherwood, 1983). Chromosomal structural rearrangements involving both inversions and translocations are very common in this mammalian taxon. Moreover intra- and/or inter-individual chromosomal variability can also be due partly to the presence of B-chromosomes. It is important to stress the evolutionary significance ofthe difference between these two types of chromosomal variability. Structural rearrangements of A-chromosomes (inversions and/or translocations) can involve the disturbance of meiotic pairing and the consequent decrease in relative fertility of structural heterozygotes. B-chromosomes, on the contrary, do not interfere with the normal development of either the mitotic or the meiotic process. In somatic mitosis, B-chromatids are irregularly scattered and consequently cells may show different chromosome numbers. During meiosis occasional pairing may occur between non-homologous B-chromosomes, as well as between A-chromosomes and B-chromosomes. Furthermore, during meiosis accumulation mechanisms lead to an increase in the number of B-chromosomes. B-chromosomes numbers can vary between individuals of the same species and even between cells of the same individual. For a general review of B-chromosomes see Jones and Rees (1982). A more detailed study of B-chromosomes in mammals has been made by Volobouev (1980, 1981). In rodents the presence of B-chromosomes has been detected in the genome of 29 species in the last 15 years (see Table 1). 94 Table l. B-chromosomes in rodents. Family Species References Chromosomes A B 2n nos. Geomyidae Thomomys bottae Thomomys umbrinus Thomornys umbrinus 76 76 78 0-12 0-30 6-12 Patton and Sherwood, 1982 Patton and Sherwood, 1982 Patton and Sherwood, 1982 Hetcromyidae Perognathus baileyi 46 0-10 Patton, 1972; 1977 Cricetidae Reithrodontomys megalot~i" Akodon Nectomys squamipes Oryzomes Tscherskia triton Dicrostonyx torquatus Microtus longicaudus Sigmodon hispidus 42 24 52-56 64 28 44-45 56 52 0-7 0-2 0-3 0-2 0-2 0-8 0-12 3-4 Blanks and Shellhammer, 1968; Shellhammer, 1969 Yonenaga et al., 1976 Maya etal., 1984 Yonenaga et al., 1976 Borisov et aL, 1978 Gileva, 1980, 1983 Judd and Cross, 1980 Zimmerman, 1970 Muridae Apodemus flavicollis Apodemus speciostt~ Apodernus giliacus Apodemus peninsulae 48 48 48 48 Golunda ellioti Mastocomys fuscus Melomys cervinipes Melomys littoralis Mus shortridgei Rattus r. rattus Rattus r. kandianus Rattus r. diardi Rattus r. frugivorus Rattus r. thai Rattus r. tanezumi Rattus fuscipes Rattus tunney Thamnomys (G.) dolichurus Uromys caudirnaculatus 54 48 48 48 46 38 40 42 38 42 42 38 42 52-54 46 0-3 0-4 0-13 0-5 0-24 0-4 0-1 0-12 0-4 0-5 0-4 0-1 0-4 0-3 0-6 0-1 0-2 0-1 4-7 6-9 Kr~l et al., 1979 Krhl, 1971 Hayata, 1973 Bekassova et al., 1980 Volobouev, 1979 Rao et al., 1979 Baverstock et aL, 1977a Baverstock et al., 1977a Baverstock et aL, 1977a Groop et al., 1973 Wahrmann and Gourevitz, 1973 Yosida, 1976 Yong and Dhalival, 1972 Ladron de Guevara et al., 1981 Groop et aL, 1970 Yosida, 1976 Baverstock et al., 1977b Baverstock et aL, 1977b Roche et al., 1984 Baverstock et al., 1976 Echirnyidae Proechimys iheringi 62 0-2 Yonennaga et al., 1985 T h e n u m b e r o f B - c h r o m o s o m e s c a n b e high in r o d e n t s , a s in A p o d e m u s p e n i n s u l a e ( 0 - 2 4 ) ( V o l o b o u e v , 1979) a n d in A p o d e m u s giliacus ( 0 - 1 3 ) ( B e k a s s o v a e t a L , 1980). C o n s i d e r a b l e v a r i a b i l i t y is f o u n d a l s o in T h o m o m y s u m b r i n u s d i s p l a y i n g t w o different p a t t e r n s , o n e with 2n --- 78 a n d 6 - 1 2 B - c h r o m o s o m e s , the o t h e r with 2n = 76 a n d 0 - 3 0 s u p e r n u m e r a r y c h r o m o s o m e s , all c o m p l e t e l y h e t e r o c h r o m a t i c (Patton & S h e r w o o d , 1982). 95 The morphology of supernumerary chromosomes varies considerably in different rodent species. In Uromys caudimaculatus 5 different types of B-chromosomes have been detected (Baverstock etal., 1976), and two types of B-chromosomes were observed in Perognatus baileyi (Patton, 1972, 1977). The occurrence of all supernumerary elements in rodents appears to be linked to an accumulation mechanism intervening during meiosis. Their presence seems to increase the chiasma frequency of A-chromosomes. The presence of B-chromosomes might have some adaptive value: two populations of Reitrodonthomys megalotis, living in different natural environments have been found displa~5ng different numbers of B-chromosomes, and different meiotic behaviour (Shellhammer, 1969). In a recent paper (Roche etal., 1984) we reported a case of inter- and intra-individual karyotype variability in a Somali population of African Climber rats attributed to the species dolichurus of the genus Thamnomys (Grammomys). Both Robertsonian fusions and B-chromosomes were found to be sources of karyotypic variability in this population. The complex taxonomic situation of these African rodents suggests an ongoing speciation process. As a matter of fact 22 morphologically very similar species were assigned to the same subgenus Grammomys. On the basis of morphological characters (Misonne, 1974), 20 of these were lumped together in the dolichurus group species (Smuts). Only T. (G.) cometes (Thomas & Wroughton) and T. (G.) rutilans (Peters) have retained their species validity. Lumping, however, into the dolichurus group is not consistent with the observed karyotypic situation: some of the 20 belong to this group although they show different karyotypes (Petter & Tranier, 1975). Thamnomys (G.) surdaster and Thamnomys (G.) buntingi have the same diploid number (2n = 52) and a similar autosomal pattern, but the X-chromosome morphology is different (Matthey, 1971), being 4 times larger in T. (G.) surdaster, than in T. (G.) buntingi. On the basis of size and colour pattern, the Somali Grammomys Climber rats previously studied by us (Roche etal., 1984), should be assigned to the ochraceus or oblitus morphs. In fact, their taxonomic position remains uncertain. The Somali Climber rats display a high degree of intra- and inter-individual karyotypic variability (2n = 56-61) associated wdth Robertsonian rearrangements and with B-chromosomes. Finally, Thamnomys (G.) gazellae shows a diploid number varying from 68 to 76 (Petter & Tranier, 1975) as a result of the presence of numerous minute autosomes which could be interpreted as B-chromosomes. Here we report the results of a cytogenetic analysis carried out on specimens from a laboratory stock of Thamnomys gazellae derived from the same central African animals previously studied by Petter and Tranier (1975) and bred for more than 10 years in the Parasitology Laboratory of the MusOe National d'Histoire Naturelle in Paris. It thus seemed worth checking the heterochromatic nature of the minute chromosomes identified by the above-named authors in order to substantiate their interpretation and to examine their meiotic behaviour. It has been possible also to describe more accurately the karyotype of T. (G.) gazellae with C- and G-banding patterns and NOR localisation. Material and methods As mentioned in the introduction, we used 10 specimens (6 males and 4 females) of Thamnomys (Grammomys) dolichurus (Smuts) sensu Misonne (1974) from the laboratory stock bred in the Music National d'Histoire Naturelle in Paris, originating from the Central African specimens whose karyotype was described as belonging to the species 'gazellae' by Tranier and Petter (1975). One male and one female were introduced into our laboratory and a breeding line was established for the present investigation. Figur e 1 shows the pedigree of the specimens analysed. Somatic metaphases were prepared from bone marrow following the usual air-drying procedure (Hsu & Patton, 1969). The meiotic preparations were obtained from testis tissues by the method of 96 Q __1 P F1 I 1::2 L Evans et al. (1964). Standard staining was carried out using 4~o Giemsa in phosphate buffer at pH 7. C-bands were produced by treatment with Ba(OH)2 as in to Bickham (1979); G-bands were induced by trypsin digestion as described by Seabright (1971). Nucleolus organiser regions (NOR) were obtained by Howell and Black's silver method (1980). 6 [ t Fig. 1. Parental relationships among the specimens investigated. Squares = males, circles = females. The female no. 2 was not analysed. Results Karyotype All specimens investigated showed intra-individual chromosomal variation, with chromosome numbers ranging from 56 to 71. Among these chromosomes, a constant fraction of A-chromosomes was identified, composed of 6 pairs of large subtelocentrics, 9 pairs of medium-sized acro- Fig. 2. Karyotype of Thamnomys (Grammomys) gazellae obtained from the mate specimen GMR 1 which displays the heteromorphism of the pair no. 6 (arrow). 97 centrics, two pairs (nos. 16 and 17) of small metacentrics and 9 pairs of small acrocentrics that we arranged in decreasing size from 2 #m to less than 1 #m. The sex chromosomes are large, the X being submetacentric and the Y acrocentric (Fig. 2). The smaller A-chromosomes elements are not easily distinguishable from the B-chromosomes in standard stained preparations. However, Cbanded preparations make it possible to distinguish the two groups being the set of B-chromosomes being "always C-positive while the small A-chromosomes appear unstained (Fig. 3). These two karyotype components can be also distinguished in the meiotic pairing patterns (Fig. 4). During meiotic diakinesis 26 autosomal bivalents and the sex-bivalent are always clearly identifiable; a variable number of unpaired small chromosomes, asymmetric bivalents or short Fig. 4. Male meiotic diakineses of T. gazellae; arrows indicate unpaired minute elements (B-chromosomes); S = Sexbivalcnt. The line at the bottom of fig. b corrcsponds to 10 #m. Fig. 3. C-banded male karyotype. Note thc constitutive hcterochromatin masses on the pericentromeric areas in the larger chromosomes, including the X and C-positive B-chromosomes. chains of minute elements are also evident.These last are typical patterns of B-chromosomes during meiosis. The sex bivalent is clearly identifiable in diakinetic preparations by a long uncoiled tail, corresponding to the unpaired part of the X-chromosome. Centromeric heterochromatic blocks are evidenced by C-banding on numerous elements (Fig. 3). Particularly heavy heterochromatic 98 blocks are evident in the 6 largest subtelocentrics and on the larger medium-sized acrocentric pairs. The B-chromosomes are always positive to C-staining. G-band analysis was performed on two animals, G M R 1, i.e. the male of the Parent generation, and G M R 6, i.e. a male of the F1 generation. (Fig. 6). G-banding permits the detection of a heteromorphism in pair No. 6, which in the G R M 1 specimen is composed of one subtelocentric and one acrocentric, while G M R 6 shows a homozygous acrocentric configuration. It has to be stressed that this chromosome pair is a carrier of nucleolar organiser regions. Four subtelocentric pairs, i.e. chromosomes Nos. 1, 2, 4, and 6 show N O R spots in the telomeric position of the short arm (Fig. 5). B-chromosome variability Fig. 5. NOR in T. gazellae. To identify the NOR chromo- somes, the same metaphase was silver stained for NORs (a), bleached with potassium hexacyanofcrrate and sodium thiosulphate and re-stained by standard bufferedGiemsa (b). Four large subteloccntricchromosomes(arrows) show silver dots on the short arm. The bar in Figure b corresponds to 10 ~m. The B-chromosomes number varies considerably within each individual. The absolute values and the relative frequency of B-chromosomes found in the 10 specimens are shown jn Table 2. These data show that, although no significant difference exists between the mean B-chromosome values found in the filial generations F1 and F2 (t = 0.25), a highly significant difference is found when the number of B-chromosomes in the father is compared with that in the offspring (t = 11.29). On the statistical evaluation of a sample comprising 4 males and 3 females, belonging to the F1 and F2 generations alone (d.f. = 67-113), the mean numbers of B-chromosomes present in animals of the two sexes (Table 3) were found to be significantly different (x males -- 10.83 + 3.1; x females 9.01 + 3.6; t + 5.22; P > 0.1~o). The significantly higher number of B-chromosomes occurring in the only male examined from the parental generation might, therefore, depend on sex rather than on the loss of heterochromatic elements in the succeeding generation. On the other hand, the sample of the parental generation (a single individual) precludes any correct analysis of the variance between parental and FI + F2 generations. However, Fisher's test analysis of variance to evaluate the homogeneity of the sample of males and females of the two filial generations did show a good homogeneity for the samples, thus supporting the significant difference between the mean numbers of B-chromosomes in the two sexes (Table 4). On the basis of an analysis performed on three males only ( G M R 1 P, G R M 6 FI and G R M 25 F2) there is no significant difference in the number of B-chromosomes from somatic 99 Fig. 6. G-banded karyotype of the male specimen GRM 1 showing heteromorphism of the pair no. 6 (arrow). The bar corresponds to 10 #m. metaphases when compared with those from spermatogonial metaphase. However, only a smN1 number of spermatogonial metaphases were observed (15 in GRM 1, 11 in GRM 6 and 13 in GRM 25). The number of B-chromosomes occurring in spermatocytes II was also evaluated using a small sample of II meiotic metaphases (20 metaphases). At a rough estimate, the numbers of B-chromosomes in this sample vary between 0 and 9 (Fig. 8). Discussion The range of variation found by us (56-71) in the number of chromosomes is lower t h a n that reported by Petter and Tranier (1975) in the wild animals from which the laborato~' stock studied by us originated. It can therefore be suggested that the number of B-chromosomes has decreased appreciably during approximately 10years of laboratory breeding (corresponding to 20-25 estimated generations), i.e. from 14-22 in 1975 to 2-17 of today. Although this finding is in agreement with the dubtful difference already reported, i.e., between the mean numbers of B-chromosomes in the father and in the filial generations, it does not agree with observations carried out on wild populations, which showed that B-chromosomes accumulation occurs from generation to generation (Patton, 1977; Borisov, 1978; Volobouev, 1979; Maia et al., 1984). Our results do not even show an increase of B-chromosomes in the germinal line vis-/t-vis the somatic line, as claimed by Gustavson and Sund (1967) for the silver fox (Vylpes vulpes fulva). Table 2. Numbers and frequencies of B-chromosomes in Tamnomys (Grammomys) gazellae. Animal Sex B-chromosomes Scored mitoses 2 3 4 5 P - Generation GRM 1 M % FI - Generation GRM 5 F GRM 6 M GRM 7 M GRM 8 F GRM 9 M G R M 10 F Total F1 ~o F2 - Generation G R M 20 F G R M 25 M G R M 26 M Total F2 ~o 2 8 10 2.3 1 1 1 6 1 6 9 1 2 18 4.2 2 3 0.7 10 2 4 1 7 12 5.8 5 2.4 7 3.4 9 2.1 6 7 8 9 10 11 12 13 14 15 3 2.6 2 1.8 4 3.5 2 1.8 4 3.5 3 2.6 6 5.3 11 9.7 16 14.16 54 47,8 7 3 9 4 8 31 7.3 1 11 1 5 1 6 25 5.8 1 3 8 7 2 12 33 7.2 7 2 6 6 13 34 8.0 13 5 12 12 14 56 13.1 3 11 8 3 24 10 59 13.8 1 9 8 4 18 6 46 10.7 1 10 8 3 6 5 33 7.7 2 5 17 4 3 1 32 7.5 3 2 5 10 2 1 23 5.4 9 8 3 20 9.8 6 1 8 5 7 10 3 20 9.8 I1 11 2 24 11.7 12 12 2 26 12.7 7 8 8 23 11.2 12 3 4 19 9.3 8 12 2 14 6.8 7 3.4 13 6.3 3 11 5.4 16 17 6 5.3 2 1.7 2 3 3 2 3 2 11 2.6 4 0.9 3 1 3 1.5 1 0.5 Total cells scored x + SD 113 13.13 +_ 3.15 14 84 78 88 83 80 427 10.11 +_ 3.31 113 67 25 205 10.04 + 3.31 Table 3. Numbers and frequencies of B-chromosomes il~ males and femates of FI and F2. Animal Sex B-chromosomes Males GRM 6 GRM 7 GRM 9 G R M 25 Total % Scored mitoses 2 3 4 2 Males Females GRM 8 G R M 10 G R M 20 Total Females 2 0,6 8 8 2.8 6 1 2 9 2.9 1 I 2 0.6 9 2 10 21 7.5 2 4 6 2.1 5 1 1 I 3 1.0 6 7 13 4.6 6 7 8 9 10 11 12 13 14 15 7 3 4 2 16 5.1 11 1 1 8 21 6.7 3 8 2 1 14 4.2 7 2 6 5 20 6.4 13 5 12 10 40 12.8 II 8 24 11 54 17.3 9 8 18 12 47 15.1 10 8 6 8 32 10.3 5 17 3 3 28 9.0 2 5 2 9 8 12 29 10.3 5 6 9 20 7,1 7 12 6 25 8.9 6 13 8 27 8.6 12 14 7 33 11.7 3 I0 11 24 8.5 4 6 12 22 7,8 3 5 7 15 5.3 4 1 12 17 6.0 10 1 8 19 6.8 9 2.9 16 3 3 3 3 12 3.8 2 2 0.7 17 2 1 3 1.0 Total cells scored x + SD 84 78 83 67 312 10.83 + 3.08 88 80 113 281 9.01 + 3.60 O 102 Table 4. Fisher's test. Variability between and within groups. Males and females considered separately. Source of variability Deviation from mean Degrees of freedom Variance Males Between groups Within groups 45.1 2836.4 3 311 15.01 9.09 F = 1.65 Females Between groups Within groups 58.2 4482.0 2 279 29.4 16.2 F = 1.81 There is an evident difference between the numbers of B-chromosomes in the two sexes, more numerous in the male than in the female. The same result was found in wild specimens of Vulpes vulpes by Renzoni and Omodeo (1972) and by Moore and Elder (1965) in foxes bred in captivity. It could therefore be postulated that the conservation and sometimes even the accumulation of B-chromosomes might take place mainly in males. If we assume that an adaptive value can be attributed to males carying a large number of B-chromosomes, these could be rapidly transmitted to the offspring. However, in laboratory conditions, as in the case of the Muridae used in the present research, all the specimens, none of which was tested for its fitness, have been used for breeding and therefore the number of B-chromosomes in the following generations depends on stochastic phenomena, and tends to level out at lower values than those found in' wild animals. This could explain why the values found in the specimens bred in our laboratory differ from those reported by Petter and Tranier (1975) for the wild animals from which our laboratory strain originated. There is no evidence whether the structural polymorphism, i.e. the pericentric inversion in chromosome no. 6, found in the laboratory strain characterized also the original wild population, or whether the rearrangement occurred in the breeding line. The karyotypes reported by Petter and Tranier (1975) were not G-banded and therefore do not make it possible to distinguish with accuracy if pair No. 6 is subtelocentric or acrocentric. However it is clear that the genome of this Variance ratio (F) species is liable to structural rearrangements. Likewise, in Somali Climber rats (Roche et aL, 1984), included in the dolichurus species group, show the same kind of polymorphism, occurring also within the individual with structural rearrangements (Robertsonian translocations) and B-chromosomes. Although it is still too early for generalized claims, the occurrence of B-chromosomes seems to be related to structural rearrangements occurring in the genome, i.e. they are the remains of centric and/or acentric fragments after chromosomal breaking associated with inversions and reciprocal translocations. The comparison of the G-banded karyotype of T. (G.) gazellae with that described by us previously (Roche et al., 1984) for the Somali Climber rats of the dolichurus group shows the occurrence of homologies in the G-banding pattern between the acrocentrics of T. (G.) gazellae and the arms of 3 metacentrics of dolichurus (Fig. 8). These involve chromosome No. 1 of dolichurus which corresponds to the fusion of Nos. 2 and 6 in gazellae; No. 3 of dolichurus corresponding to acrocentrics Nos. 8 and 9 ofgazellae; and metacentric No. 4 in dolichurus corresponding to Nos. 1 and 11 of gazellae (Fig. 7). Metaeentric No. 5 ofdolichurus displays G-bands on the long arm which are homologous with acrocentric No. 7 of gazellae. Other G-band homologies between autosomes of the two species remain unclear. On the other hand, the G-band pattern of the X-chromosomes of the two species is identical. Comparison of the diploid numbers of the subspecies assigned by Misonne's revision (1974) to 103 Fig. 7. Two 2nd meiotic metaphases showing 36 (a) and 31 (b) chromosomes, respectively. The line on the bottom of Figure a corresponds to 10/~m. the dolichurus group shows that the Ivory Coast butigi and the Katanga surdaster share a 2n = 52 karyotype. Higher numbers are found in the Central African Republic gazellae and in the Somali dolichurus s.l. ( 2 n = 6 8 - 7 6 and 2n = 56-61, respectively). However, if these numbers are corrected by subtracting the B-chromosomes set, a similar diploid number, i.e. 2n = 52-54, is shared by the Western- and Central-African populations. The subsequent occurrence of Robertsonian translocations further reduces the number of chromosomes in the Somaliland population, which is the easternmost of all the species. Therefore, despite the apparent great diversity suggested by the number of chromosomes, karyological evidence actually points to a certain degree of homogeneity inside the doliehurus group, which Fig. 8. G-band comparison between the metacentrics of T. dolichurus from Somaliland (middle column, S) and the acrocentrics of 1'. gazellae from Central African Republic (lateral columns, G). Numbers refer to the pair identifications proposed by Roche et at. (1984) for dolichurus and in the present paper for gazellae. The X-chromosomes of the two species are also compared. The line corresponds to 10 ttm. is broken by ongoing processes of karyotypic rearrangement accompanied by a conspicuous intra-individual variability related to B-chromosomes. 104 Acknowledgements The authors are grateful to Prof. Francis Petter of the Musde National d'Histoire Naturelle in Paris for supplying this interesting material. The financial help of the Italian National Research Council (CNR) and the Ministry of Education (M.P.I.) is also gratefully acknowledged. References Baverstock, P. R., Watts, C. H. S. & Hogarth, J.T., 1976. Heterochromatin variation in the Australian rodent Uromys caudimaculatus. Chromosoma 57: 397-403. Baverstock. P. R., Watts, C. H. S., Hogarth, J. T., Robinson, A. C. & Robinson, J. F., 1977a. Chromosome evolution in Australian rodents. I. The Pseudomyinae, the Hydromyinae and the Uromys/Melomys group. Chromosoma 61: 95-125. Baverstoek, P. R., Watts, C. H. S., Hogarth, J. T., Robinson, A. C. & Robinson, J. F., 1977b. Chromosome evolution in Australian rodents. II. The Rattus group. Chromosoma 61 : 227-241. Bekassova, T. S., Vorontsov, N. N., Korobitsyna, K.V. & Korablev, N. B., 1980. B-chromosomes and comparative karyology of the mice of the genus Apodemus. Genetica 52/53: 33-43. Bickham, J. W., 1979. Banded karyotypes of 11 species of American bats (genus Myotis). Cytologia 44: 789-797. Blanks, G.A. & Shellhammer, H.S., 1968. Chromosome polymorphism in California population of harvest mice, J. Mammal. 49: 726-731. Borisov, J.M., Korablev, V.P., Karatavtseva, I.V., Lyapunova, E. A. & Vorontsov, N.N., 1978. Additional chromosomes in rat-like hamster and its systematical status. Reports of 2nd meeting of All-union Theriological Society of USSR. pp. 13-14. Publishing house 'Nauka', Moscow. (In Russian, quoted by Volobouev, 1981). Evans, E. P., Breckon, G. & Ford, C. E., 1964. An air-drying method for meiotic preparations from mammalian testes. Cytogenetics 3: 289-294. Gileva, E. A., 1980. Chromosomal diversity and an aberrant genetic system of sex determination in the arctic lemming Dierostonyx torquatus Pallas, 1779. Genetica 52/53: 99-103. Gileva, E.A., 1983. A contrasted pattern of chromosome evolution in two genera of lemmings, Lemmus and Dicrostonyx (Mammalia, Rodentia). Genetica 60: 173-179. Gropp, A., Marshall, J., Flatz, G., Olbrich, M., Manyanondha, K. & Santadusit, A., 1970. Chromosomen Polymorphismus durch taberz~.hlige Autosomen. Beobachtungen an der Hausratte (Rattus rattus). Z. SSugetierk. 35: 363-371. Gropp, A., Marshall, J. & Markvong, A., 1973. Chromosomal findings in the spiny mice of Thailand (genus Mus) and occurrence of a complex intraspecific variation in M. shortridgei. Z. S~ugetierk. 38: 159-168. Gustavsson, I. & Sundt, C. O., 1967. Chromosome elimination in the evolution of the silver fox (Vulpes fulvus Desm.). J. Hered. 58: 75-78. Hayata, J. 1973. Chromosomal polymorphism caused by supernumerary chromosomes in the field mouse, Apodcmus giliacus. Chromosoma 42: 403-414. Howell, W.M. & Black, D.A., 1980. Controlled silverstaining of nucleolus organiser regions with a protective colloidal developer: a 1-step method. Experientia 36: 1014-1015. Hsu, T. C. & Patton, J. L., 1969. Bone marrow preparations for chromosome studies. In: Comparative mammalian cytogenetics. K. Benirschke (ed.). pp. 454-460 Springer, Berlin, Heidelberg, New York. Jones, R.N. & Rees, H., 1982. B-chromosomes. pp. 266. Academic Press, London. Judd, S. R. & Cross, S. P., 1980. Chromosomal variation in Microtus longicaudus (Merriam). Murrelet 61: 2-5. In: Kral, B., 1971. Chromosome characteristics of certain murinc rodents (Muridae) of the Asiatic part of the USSR. Zoologicke Listy 20: 331-347. Kral, B., Zima., J., Herzig-Straschil, B. & Sterba, O., 1979. Karyotypes of certain small mammals from Austria. Folia Zoologica 28: 5-11. Ladron de Guevara, R. G. & Diaz de la Guardia, R., 1981. Frequency of chromosome polymorphism for pericentric inversions and B-chromosomes in Spanish populations of Rattus rattus frugivorus. Genetica 57: 99-103. Maia, V., Yonenaga-Yassuda, Y., Freitas, T. R. O., Kasahara, S., Sune-Mattevi, M. & Oliviera, L.F., 1984. Supernumerary chromosomes, Robertsonian rearrangement and variability of the sex chromosomes in Nectomys squamipes (Cricetidae, Rodentia). Genetica 63: 121-128. Matthey, R., 1971. Dimorphisme sexuel X et caryotype de Grammomys surdaster Th. & Wrough (Mammalia, Muridae). Boil. Zool. 38: 183-186. Misonne, X., 1974. Part 6. Order Rodentia, main text. In: The Mammals of Africa: an identification manual. Parts 1-15 (1971-1977). J. Meester & H.W. Setzer (eds.). Smithsonian Inst. Press, Washington. Moore, J. W. & Elder R. L., 1965. Chromosomes of the fox. J. Herod. 56: 142-143. Patton, J. L., 1972. A complex system of chromosomal variation in the pocket mouse Perognathus baileyi Mcrriam. Chromosoma 36: 241-256. Patton, J.L., 1977. B-chromosome system in the pocket mouse Perognathus baileyi: meiosis and C-band studies. Chromosoma 60: 1-14. Patton, J. L. & Sherwood, S. W., 1982. Genome evolution in the Pocket gophers (genus Thomomys). 1. Heterochromatin variation and speciation potential. Chromosoma 85: 149-162. Patton, J. L. & Sherwood, S. W., 1983. Chromosome evolu- 105 tion and speciation in rodents. Ann. Rev. Ecol. Syst. 14: 139-158. Petter, F. & Tranier, M., 1975. Contribution ~t l'6tude des Thamnomys du groupc dolichurus (Rongeurs. Murid6s). Syst6matique et caryologie. Mammalia 39: 405-414. Rao, K. S., Aswathanarayana, N. V. & Prakash, K. S., 1979. Supernumerary (B) chromosomes in the Indian bush rat. Golunda ellioti (Gray). Mamm. Chrom. Newsl. 20: 79. Renzoni, A. & Omodeo. P., 1972. Polymorphic chromosome system in the fox. Caryologia 25: 173-187. Roche, J., Capanna, E., Civitelli, M. V. & Ceraso, A., 1984. Caryotypes des rongeurs de Somalie. 4. Premi6re capture de rongeurs arboricoles du sous genre Grammomys (genre Thamnomys, Murid6s) en R6publique de Somalie. Monitore Zool. Ital. (N.S.) Suppl. 7: 259-277. Seabright, M. A., 1971. A rapid banding technique for human chromosomes. Lancet 2: 971-972. Shellhammer, H. S., 1969. Supernumerary chromosomes of the harvest mouse, Reithrodontomys megalotis. Chromosoma 27: 102-108. Volobouev, V. T., 1979. Karyological analysis of three populations of Asiatic forest mouse Apodemus peninsulae (Rodentia, Muridae) from Siberia. Doklady Acad. Nauk USSR 248: 1452-1454. Volobouev, V.T., 1980. The B-chromosomc system of mammals. Genetica 52/53: 333-337. Volobouev, V.T., 1981. B-chromosome system of the Mammals. Caryologia 34: 1-23. Wahrman, J. & Gourevitz, P., 1973. The chromosome biolo~' of the 2n = 38 black rat, Rattus rattus. Chromosomes Today 4: 433-434. John Wile)' & Sons, New York and Toronto. Yonenaga, Y., Frota-Pessoa, O., Kasahara S. & Cardoso de Almeida, E.J., 1976. Cytogenetic studies on Brasilian rodents. Ciencia e Cultura 28: 202-211. Yonenaga-Yassuda, Y., De Souza, M.J., Kasahara, S., L'Abate, M. & Tien Chu, H., 1985. Supernumerary system in Proechimys iheringi iheringi (Rodentia, Echimidae), from the State of Sao Paulo, Brazil. Caryologia 38: 179-194. Yong, H.S. & Dhalival, S.S., 1972. Supernumerary (B) chromosomes in the Malayan house rat, Rattus rattus diardii (Rodentia, Muridae). Chromosoma 36: 256-263. Yosida, T. H., 1976. Population survey of B-chromosomes in black rats. Ann. Rep. Nat. Inst. Genetics 26: 33-34. Zimmerman, E. G., 1970. Kao'ology, Systematics and chromosome evolution in the rodent genus Sigmodon. Publ. Mich. State Univ. Museum, Biol. Ser. 4: 389-454.