Michael W. Haiduk

Chromosomal evolution in African megachiroptera: G- and C-band assessment of the magnitude of change in similar standard karyotypes

Differential staining (G- and C-banding) is used to assess magnitude of chromosomal variation in eight species of African megachiropterans. These data are compared with those suggested by standard karyotypic studies, and it is concluded that in this example standard methods underestimate chromosomal variation by a factor greater than 4.5. The implication of these data is that models constructed from patterns of chromosomal evolution in standard karyotypes need to be evaluated in light of in-depth studies based on G- and C-bands.

Evolution and Phylogenetic Significance of Ribosomal Gene Location in Chromosomes of Squamate Reptiles

Using in situ hybridization with a biotin-labeled probe, we determined the chromosomal location of ribosomal genes in 56 species of squamate reptiles, including representatives of nine major taxa. Where possible, these data were examined in a phylogenetic context, and in several cases they provided phylogenetically useful shared derived character states. The ribosomal genes in Sceloporus variabilis are found on a single pair of microchromosomes, which seems to be primitive for the phrynosomatids. In the remainder of species of Sceloporus we examined, the ribosomal genes are found on the long arm of pair 2. We also found that Holbrookia, Cophosaurus, and Callisaurus share a derived condition not found in Uma. Two species of the viperid genus Agkistrodon share a condition that may be derived relative to other pit vipers. A third species of Agkistrodon differs from all other reptiles we examined in that ribosomal genes are located on the sex chromosomes. Location of rDNA also provides systematic information in several other clades of squamates.

Cladistical analysis of primitive G-band sequences for the karyotype of the ancestor of the Cricetidae complex of rodents

Homologous segments identified by G-banding sequences of chromosomes of Peromyscus boylii, Neotoma micropus, Oryzomys capito, (Family Cricetidae) Rattus norvegicus, Melomys burtoni, and Apodemus sylvaticus (Family Muridae) were used to hypothesize a chromosomal condition for the cricetid ancestor. A critical assumption in proposing the primitive G-banding sequences for a given chromosome is that if the outgroup and ingroup taxa have a specific sequence, then the ancestor of the ingroup taxa also had that same sequence. Using this methodology, (chromosome numbers refer to proposed homology to the standardized karyotype for Peromyscus), we propose that: (1) the primitive banding pattern of chromosome 1 was identical to that of Neotoma; (2) the primitive patterns of chromosomes 2, 3, 4, 6, 7, 8, 9, 10, 11, and 12 were primitive banding patterns of 5 and 13 were undetermined; (4) a major portion of the banding patterns of 14 and X were present in the ancestral karyotype. Only the largest 14 autosomes and X were examined because the smaller elements had insufficient G-band definition to ensure reasonable accuracy. The karyotype ancestral to that of Peromyscus, Neotoma, and Oryzomys may be as above and the banding patterns of 5, 13, and 14 were acrocentric and identical to those shown for Peromyscus, Neotoma, and Oryzomys (Fig. 1). In the primitive karyotype, heterochromatin (C-band material) was probably limited to the centromeric regions. If the primitive karyotype is as described above, then it is possible to determine the direction, type, and magnitude of chromosomal evolution evident in the various cricetid lineages. Based on the available data, radiation from the ancestral cytotype is characterized by a nonrandom distribution of types of chromosomal changes. Within many genera, more rearrangements occur in the 14 largest autosomal chromosomes of some congeneric species than distinguish the proposed primitive conditions for the genera Peromyscus, Neotoma, and Oryzomys. It would appear that the extensive morphological radiation from the primitive cricetid ancestor as indicated by the presence of over 100 surviving genera within the family, was not accompanied by extensive karyotypic changes. The magnitude of chromosomal variation that accompanies speciation in these genera appears to range from no detectable chromosomal evolution to a radical reorganization of the genome.

Banded Karyotypes of Six Taxa of Kinosternid Turtles

C- and G-banding analyses were performed on representatives of three genera of kinosternid turtles (Kinosternon, Sternotherus, Staurotypus), representing both subfamilies and all known chromosomal variation in the family. Those species C-banded showed that constitutive heterochromatin was restricted to the centromeric regions of the chromosomes, but the number of pairs having heterochromatic regions differed among species. Species of Kinosternon from the southeastern United States were more similar to Sternotherus in the amount and distribution of heterochromatin than they were to Kinosternon from Mexico. G-band data showed that chromosomal change has been minimal since the divergence of the three genera. Banding patterns are distinctive for most macrochromosomes of each species, and almost all homologous arms can be identified among the three genera. We conclude that the karyotype characteristic of kinosternine turtles was derived from the Staurotypus karyotype by a pericentric inversion in the second largest chromosome pair and by a fission in one pair of group B chromosomes. This accounts for the differences in chromosome morphology and diploid number between the two subfamilies.

Chromosomal Homologies and Evolution of Testudinoid Turtles with Emphasis on the Systematic Placement of Platysternon

The karyotype of Platysternon (2n = 54) is compared to members of the Chelydridae, Emydidae and Kinosternidae and is found to be the most similar to the primitive batagurine emydid condition (2n = 52). The batagurine karyotype can be derived from that of Platysternon (or the reverse) by one event whereas several are required to derive Platysternon from either Chelydra or Macroclemys. Moreover, derived chromosomal characters are shared between Platysternon and emydids. It is proposed that the Platysternidae (Gray) may be valid, including only Platysternon. Chromosomal data are also used to develop a phylogeny of the Testudinoidea in which the Chelydridae is found to be an early offshoot of the emydid-testudinid line and the Platysternidae is derived from a later form ancestral to the Emydidae and Testudinidae.