Ira F. Greenbaum

Cytosystematic Value of Chromosomal Inversion Data in the Genus Peromyscus (Rodentia: Cricetidae)

G-band chromosomal data are presented for seven species of subtropical and tropical deer mice: Peromyscus banderanus, P. guatemalensis, P. gymnotis, P. mexicanus, P. yucatanicus, P. zarhynchus , and P. lepturus. P. banderanus retains the primitive karyotype for the genus with only chromosomes 1, 22, and 23 biarmed. Species examined from the mexicanus group ( P. guatemalensis, P. gymnotis, P. mexicanus, P. yucatanicus , and P. zarhynchus ) are characterized by a single karyotype with chromosomes 1, 2, 3, 9, 22, and 23 biarmed. P. lepturus has chromosomes 1, 2, 3, 5, 6, 7, 9, 10, 15, 22, and 23 biarmed. Autosomal heterochromatin (C-band positive material) was restricted to the centromeric regions in each of the seven species examined. A most parsimonious cladogram was constructed from the chromosome banding data of these and 19 other species. Due to the high frequency of apparently convergent inversions of chromosome 6, this chromosome was eliminated as a character in the cladistic analysis. Additionally, a synthetic phylogenetic analysis incorporating chromosomal and subgeneric taxonomic inferences for the 26 species is presented. Within the genus Peromyscus , chromosome banding data are viewed as most reliable at the species group level (in certain cases) and as indicators of potential areas of discrepancy in intrageneric taxonomy.

Evidence for heterosynaptic pairing of the inverted segment in pericentric inversion heterozygotes of the deer mouse (Peromyscus maniculatus)

Silver-stained pachytene cells of male deer mice, Peromyscus maniculatus, which were heterozygous for a naturally occurring pericentric inversion of chromosome 6, were analyzed by light microscopy. The presence of the terminally positioned inversion, involving approximately 30% of the length of chromosome 6, was detected by G-banding. Within the inversion, C-band-positive heterochromatin was restricted to the centromeric region. Silver-staining of spermatocytes revealed the synaptonemal complexes (SCs) of the autosomal bivalents and the X-Y chromosome association. Pachytene cells were scored for the presence of inversion loops corresponding to the pericentric inversion of chromosome 6. Possible loop 6 configurations were detected in less than 1% of the cells examined, the vast majority of cells having regularly formed autosomal SCs similar to those reported for homomorphic chromosome pairs in other rodent species. It appears, therefore, that in these mice the inverted region of chromosome 6 was heterosynaptic throughout pachytene. Heterosynapsis is hypothesized as a mechanism which might obviate the production of duplication and deletion chromatids expected from crossing-over in pericentric inversion loops. The observation of heterosynaptic pairing in the inverted segment and the hypothesis of inversion heterosynapsis as a mechanism averting gametic loss are consistent with the widespread occurrence of pericentric inversion polymorphisms in P maniculatus and the apparent failure of pericentric inversions to genetically isolate populations of this species.

The mechanism of autosomal synapsis and the substaging of zygonema and pachynema from deer mouse spermatocytes

Surface-spread and silver-stained spermatocytes of Peromyscus maniculatus and P. sitkensis were analyzed in order to develop criteria for the recognition of meiotic substages from early zygonema through early diplonema. The continuous sequence of changes in the morphology of the autosomal axes (lateral elements) of the synaptonemal complexes (SC), sex chromosome axes, and nucleoli enabled the recognition of three substages of zygonema and five of pachynema. The proposed system of subdivision is compatible with descriptions of comparable data from Chinese hamsters and laboratory mice with differences being primarily associated with the timing of sex chromosome synapsis and desynapsis. Within the substages, cytogenetically important details of the synaptic mechanism in deer mice were noted. Autosomal synaptic initiation in deer mice is apparently uniterminal, involving the distal (noncentromeric) end of the homologs. Subsequent pairing is unidirectional towards the centromeric end. Additionally, during mid and late zygonema the homologous axes of late pairing regions of some autosomes were characterized by substantial length differences. These lateral element length differences were not maintained into pachynema and it is hypothesized that differences in the amount of material in the heterochromatic short arms of these species may be subject to synaptic adjustment.

Systematic and Taxonomic Implications of Karyotypic, Electrophoretic, and Mitochondrial-Dna Variation in Peromyscus from the Pacific Northwest

Chromosomes, allozymes, and mitochondrial DNA (mtDNA) were analyzed to examine the taxonomic and systematic relationships of coastal Peromyscus from northern Washington to southern Alaska. All three datasets indicate that Peromyscus from this region constitute two distinct groups. One group comprises P. oreas, P. sitkensis , and several currently recognized subspecies of P. maniculatus including P. m. algidus, P. m. hylaeus, P. m. keeni, P. m. macrorhinus , and P. m. prevostensis . The second group comprises only populations of P. m. austerus . The former group is differentiated from the latter by number of autosomal arms, allele-frequency differences, and mtDNA haplotypes. This dichotomy in karyotype, and the level of allozymic and mtDNA divergence between the groups suggest that the groups constitute distinct species. We recommend that P. oreas, P. sitkensis, P. maniculatus algidus, P. m. hylaeus, P. m. keeni, P. m. macrorhinus , and P. m. prevostensis be recognized under the specific epithet of Peromyscus keeni .

GENETIC INTERACTIONS BETWEEN HYBRIDIZING CYTOTYPES OF THE TENT-MAKING BAT (URODERMA BILOBATUM )

The vast majority of described hybrid zones between cytotypes (conspecific populations which differ karyotypically) are narrow and generally involve organisms of restricted vagility. Theories regarding the evolutionary genesis and fate of such zones most often have been hypothesized to be a direct function of low vagility and its effects upon population parameters. Recent investigations (Baker et al., 1972; Baker et al., 1975; Baker, 1979, 1981), describe an extensive zone of intergradation between hybridizing cytotypes of the tent-making bat, Uroderma bilobatum (Phyllostomatidae: Stenoderminae). Uroderma bilobatum convexum (2n = 38) and U. b. davisi (2n = 44) come into contact along the coast of the Golfo de Fonseca in Honduras and karyotypic intermediates range from west central Guatamala to northwestern Nicaragua. A culmination of the chromosomal data available for these cytotypes is presented in the companion paper to this manuscript (Baker, 1981). Gand C-band data (Baker, 1979) reveal the uniqueness of the individual chromosomal differences between the two parental races which enable relatively unambiguous identification of hybrids and backcross individuals. Baker (1981) characterized the nature of contact between parental types, distribution of hybrids and backcrosses within the zone, and success of each of the individual rearrangements in populations dominated by other cytotype. He concluded that negative heterosis against hybrids and backcrosses is not severe and that limited hybridization supplemented by backcrossing and dispersal produce long lasting chromosomal polymorphism over a broad zone. The present study was designed to use electrophoretic techniques to determine the population genetic characteristics of U. b. davisi and U. b. convexum and the nature of genetic interaction across the broad contact zone between them.

Cytogenetic nomenclature of deer mice, Peromyscus (Rodentia): revision and review of the standardized karyotype

A revision of the standardized karyotype of deer mice (Peromyscus) is presented. This revision addresses short-comings of the original standardization, contains a substantial increase in the number of G-band markers and provides a nomenclature for the G-bands of each autosome and the X chromosome. Using the revised standardized karyotype, we specify the particular G-bands or patterns that identify each chromosome and catalog the more problematic chromosome identifications and likely misidentifications. For each chromosome, we present an overview of previously reported variation in euchromatic arrangement and heterochromatic constitution. We then review previous applications of the standardized karyotype and summarize the predominant findings from cytogenetic and cytosystematic studies of Peromyscus and related taxa.

CHROMOSOME EVOLUTION IN THE IGUANID LIZARD SCELOPORUS GRAMMICUS . II. ALLOZYME VARIATION

In the previous paper Sites (1983) described several chromosome polymorphisms within three cytotypes of Sceloporus grammicus. It was concluded that evolution of these chromosomal rearrangements was probably not causally related to speciation. Several of the models discussed yielded specific predictions about population structure that are amenable to empirical testing with an independent data set. This study reports on allozyme variation within and among the same three cytotypes (2n = 32, 2n = 34, and 2n = 36) in the same geographic area as those studied by Sites (1982a) and focuses on three major points of interest: (1) genetic divergence within and among these cytotypes; (2) population subdivision as determined by the distribution of allele frequencies across the population samples; and (3) levels of heterozygosity maintained in each of these cytotypes.

Mitochondrial-DNA Analysis of the Systematic Relationships within the Peromyscus maniculatus Species Group

Both parsimony and distance-based phylogenetic analyses of sequence data from three mitochondrial DNA (mtDNA) genes (ND3, ND4L, and ND4) revealed that, as currently recognized, the Peromyscus maniculatus species group is polyphyletic. This apparent poly-phyly is resolved when P. slevini is removed from the species-group, thus limiting the group to P. maniculatus, P. keeni, P. polionotus, P. sejugis , and P. melanotis . Additionally, the phylogenetic analysis revealed that P. m. coolidgei is closer to P. sejugis than to the other subspecies of P. maniculatus examined. The systematic integrity of P. maniculatus is further complicated by the sister-species relationship between P. keeni and P. sejugis-P. m. coolidgei . These relationships suggest either a high degree of lineage sorting within P. maniculatus or that some of the currently recognized subspecies may be distinct species.

CHROMOSOMAL HOMOLOGY AND DIVERGENCE BETWEEN SIBLING SPECIES OF DEER MICE: PEROMYSCUS MANICULATUS AND P. MELANOTIS (RODENTIA, CRICETIDAE)

From a chromosomal standpoint, speciation in the Peromyscus maniculatus complex has been accompanied by considerable change in the number of arms in the autosomal complement. This report is concerned with a determination of the chromosomal homologies and types of chromosomal rearrangements between two members of this complex, P. melanotis and P. maniculatus, as determined by Gand Cband comparisons. This study was undertaken to document the level of chromosomal evolution that is characteristic of some sibling species as well as to provide some insight into the evolutionary history of these two species. The diploid number for all species of Peromyscus thus far described is 48. Numerous authors have, however, documented extensive chromosomal variation dealing with the number of chromosome arms in Peromyscus (Hsu and Arrighi, 1966; Ohno et al., 1966; Singh and McMillan, 1966; Sparkes and Arakaki, 1966; Hsu and Arrighi, 1968; Arakaki et al., 1970; Te and Dawson, 1971; Bradshaw and Hsu, 1972; Lee et al., 1972; Bowers et al., 1973; Pathak et al., 1973; Arrighi et al., 1976; Schmidly and Schroeter, 1974; Murray and Kitchin, 1976). Hsu and Arrighi (1968) described the karyotype for 19 species of Peromyscus and reported both interand intraspecific variation of 56-96 in the total number of chromosome arms. Bradshaw and Hsu (1972) documented chromosomal variation in 15 subspecies of P. maniculatus and suggested that where dual modes were evident such as in P. m. rufinus of southeastern Arizona that these populations might be representative of another species (e.g., P. melanotis). Bowers et al. (1973) and Bowers (1974) have shown by karyotypic, electrophoretic, and breeding data that this is indeed the case. It has been most convenient and practical to describe chromosomal variation in Peromyscus in terms of the number of biarmed versus the number of acrocentric elements in the autosomal complements, although in some cases it is difficult to separate the two categories. Bradshaw and Hsu (1972) chose to describe variation in P. maniculatus in terms of the number of biarmed elements in the autosomal complement, whereas Bowers et al. (1973) chose to employ the number of autosomal acrocentrics for their descriptions. We have chosen the latter for our discussion. A summary of the current view of chromosomal variation in P. maniculatus is presented in our discussion.

Identifying chromosomal fragile sites from individuals : a multinomial statistical model

The inability to identify fragile sites from data for single individuals remains the major obstacle to determining whether these chromosomal loci are predisposed to cancer-causing and evolutionary rearrangements. We describe a novel statistical model that is amenable to data from single individuals and that establishes site-specific chromosomal breakage as nonrandom with respect to the distribution of total breakage. Our method tests incrementally smaller subsets of the data for homogeneity under a multinomial model that assigns equal probabilites to a maximal set of nonfragile sites and unrestricted probabilities to the remaining fragile sites with significantly higher numbers of breaks. We show how standardized Pearsons chi-square (X2) and likelihood-ratio (G2) statistics can be appropriately used to measure goodness-of-fit for sparse contingency (individual-based) data in this model. A sample application of this approach indicates extensive variation in fragile sites among individuals and marked differences in fragile-site inferences from pooled as opposed to per-individual data.