Genealogical concordance and the specific status of Peromyscus sejugis.

Peromyscus sejugis, a peripheral isolate of Peromyscus maniculatus, is a threatened taxon endemic to 2 small islands in the Sea of Cortés. Although its insularity makes the specific recognition of P. sejugis inherently problematic, resolution of this problem has important conservation implications. To evaluate the specific validity and evolutionary history of P. sejugis, we compared sequence variation (ND3/ND4L/ND4) in mtDNA for both island populations of P. sejugis with that for 8 populations of P. maniculatus from mainland Baja California. Each island population of P. sejugis had a single haplotype (0.7% sequence divergence), whereas 11 different haplotypes (mean sequence divergence = 0.68%) were obtained for the populations of P. maniculatus. The mean sequence divergence between the populations of the 2 species was 2.0%. Nested clade analysis supports the conclusion that P. sejugis is an insular isolate of P. maniculatus from mainland Baja California. Although our analysis confirms a low level of mtDNA divergence between P. sejugis and P. maniculatus from Baja California, the genealogical concordance of morphological, chromosomal, microsatellite, and mtDNA haplotype distinctiveness supports the conclusion that the 2 island populations of P. sejugis constitute independent evolutionarily significant units and together represent a phylogenetic species distinct from the P. maniculatus from Baja California.

Microsatellite variation and evolution in the Peromyscus maniculatus species group.

Variation at 12 pure-repeat dinucleotide microsatellites from Peromyscus maniculatus was analyzed for samples of all species in the P. maniculatus species group and P. leucopus. Except for one locus (Pml08) that amplified a product only for P. maniculatus, these microsatellites yielded reliable estimates of variation across these species; per-locus polymorphism and allele-size distribution were not significantly different among or between any of the species sampled from mainland populations. Significantly lower levels of variation and the distribution of alleles in the two populations of the insular-endemic P. sejugis were consistent with the expectation of substantial founder effect and suggest a lack of recent gene flow between these populations. Phenetic analyses of genetic distances based on shared allele frequencies uniformly produced well-supported trees that were entirely concordant with the a priori corroborated relationships within the P. maniculatus species group; this result was not obtained with analyses of a genetic distance computed from differences in allele sizes. The microsatellite data do not support the hypothesis that P. sejugis should be considered conspecific with P. maniculatus but yield a strongly supported sister-group association between P. sejugis and P. keeni.

Probability models and the applicability of statistical procedures in the identification of chromosomal fragile sites.

Böhm et al. (1995, Human Genetics 95, 249-256) introduced a statistical model (named FSM--fragile site model) specifically designed for the identification of fragile sites from chromosomal breakage data. In response to claims to the contrary (Hou et al., 1999, Human Genetics 104, 350-355; Hou et al., 2001, Biometrics 57, 435-440), we show how the FSM model is correctly modified for application under the assumption that the probability of random breakage is proportional to chromosomal band length and how the purportedly alternative procedures proposed by Hou, Chang, and Tai (1999, 2001) are variations of the correctly modified FSM algorithm. With the exception of the test statistic employed, the procedure described by Hou et al. (1999) is shown to be functionally identical to the correctly modified FSM and the application of an incorrectly modified FSM is shown to invalidate all of the comparisons of FSM to the alternatives proposed by Hou et al. (1999, 2001). Last, we discuss the statistical implications of the methodological variations proposed by Hou et al. (2001) and emphasize the logical and statistical necessity for fragile site identifications to be based on data from single individuals.

Fragility in the 14q21q translocation region

Aphidicolin (APC)-induced chromosomal breakage was analyzed for women representing three generations of a single family and carrying a Robertsonian translocation rob(14q21q). Fluorescence in situ hybridization (FISH) analysis confirmed the dicentric constitution of the derived chromosome and indicated the absence of beta-satellite signal at the translocation region. Per-individual analysis of metaphases from APC-treated peripheral blood lymphocyte cultures identified significantly nonrandom chromosomal breakage at the translocation region in all three individuals examined. The APC-inducible fragility at the 14q21q translocation region suggests that this rearrangement was the result of chromosomal mutation at fragile site(s) in the progenitor chromosomes, or that this fragility was the result of the fusion of nonfragile progenitor chromosomes

CYTOGENETIC ANALYSIS OF CHROMOSOMAL INTERMEDIATES FROM A HYBRID ZONE BETWEEN TWO CHROMOSOME RACES OF THE SCELOPORUS GRAMMICUS COMPLEX (SAURIA, PHRYNOSOMATIDAE).

Underdominance for chromosomal rearrangements is the central assumption of several models of chromosomally based speciation including the cascade model, proposed for the Sceloporus grammicus complex. Several cytotypes of the S. grammicus complex hybridize at localities in central México. A hybrid zone between two of the most chromosomally divergent races (= cytotypes) of S. grammicus (F5, 2n = 34 and FM2, 2n = 44-46) was examined to assess the meiotic effects of heterozygosity at multiple chromosomes. Meiosis was examined in males heterozygous for "simple" Robertsonian fissions at chromosomes 1, 3, 4, and 6 and/or a pericentric inversion at chromosome 4. Analysis of synaptonemal complexes and chromosomal configurations at diakinesis showed trivalent formation in fission heterozygotes and heterosynapsis (lack of reverse-loop formation) in an inversion heterozygote. Analysis of metaphase II configurations revealed primarily balanced segregation and low levels of nondisjunction regardless of chromosomal background. The lack of underdominance associated with "simple" fission heterozygosity in this narrow hybrid zone contradicts the key premise of most chromosomally based models of speciation.

DYNAMICS OF A NOVEL CHROMOSOMAL POLYMORPHISM WITHIN A HYBRID ZONE BETWEEN TWO CHROMOSOME RACES OF THE SCELOPORUS GRAMMICUS COMPLEX (SAURIA, PHRYNOSOMATIDAE).

Several chromosome races of the mesquite lizard, Sceloporus grammicus complex, hybridize at localities in central Mexico. In most cases, the hybridizing populations are delineated by centric fissions at one or more of the macrochromosomes. One notable exception is the Tulancingo hybrid zone between the F5 and FM2 cytotypes. In addition to fission and/or inversion differences at chromosomes 1, 3, 4, and 6, these races differ by a complex rearrangement of chromosome 2, which carries the nucleolus-organizer region in this species. The meiotic consequences of heterozygosity at this chromosome were examined in males to assess the potential for this chromosome to contribute to the dynamics of the hybrid zone. Chromosomal analysis revealed several putative F1 hybrids and confirmed the production of nonparental chromosomal morphologies through recombination. Pachytene analysis revealed meiotic pairing difficulties associated with chromosome 2 in males heterozygous for the parental chromosomal morphologies. Significant aneuploidy is expected because of random disjunction of the chromosome-2 elements. As a result, these males likely suffer reduced fertiliity and fitness. In contrast, males heterozygous for recombinant chromosomal morphologies displayed low levels of meiotic irregularities and presumably exhibit higher fertility than individuals heterozygous for parental morphologies. It is hypothesized that the recombinant phenotypes facilitate gene flow between the F5 and FM2 cytotypes.