The natural habitats of many species have become fragmented into small "islands," principally by human activities. In this paper we discuss the long-term genetic and evolutionary consequences of fragmentation as inferred from studies on populations that have undergone natural habitat fragmentation in the Ozark Mountains. The Ozarks are the highest land formation found in the midwestern United States. Because of the absence of major geographical barriers around the Ozarks, plants and animals from diverse parts of the continent have been able to invade the area during post-Pleistocene climatic periods. Many of these invasions were short-lived, but the geological and topographical complexity of the Ozarks provided numerous relictual habitats. As a

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Analyses of mitochondrial (mt) DNA control region sequences from 175 leatherback turtles Dermochelys coriacea from 10 nesting colonies revealed shallow phylogenetic structuring of maternal lineages on a global scale. Eleven haplotypes were observed, and mean estimated sequence divergence, p = 0.00581, is much lower than the deepest nodes reported in global mtDNA surveys of the green turtle Chelonia mydas, loggerhead Caretta caretta, and ridley turtles Lepidochelys spp. The leatherback turtle is the product of an evolutionary trajectory originating at least 100 million years ago, yet the intraspeci®c phylogeny recorded in mitochondrial lineages may trace back less than 900 000 years. The gene genealogy and global distribution of mtDNA haplotypes indicate that leatherbacks may have radiated from a narrow refugium, possibly in the Indian±Paci®c during the early Pleistocene glaciation. Analysis of haplotype frequencies revealed that nesting populations are strongly subdivided globally (F ST = 0.415), and within ocean basins (F ST = 0.203±0.253), despite the leatherback's highly migratory nature. Within the Atlantic signi®cant differences in haplotype frequency distributions and Nm values < 2 are observed in pairwise comparisons between St. Croix (U.S. Virgin Islands) and mainland Caribbean populations, and between Trinidad and the same mainland populations. These ®ndings provisionally support the natal homing hypothesis for leatherback turtles, although several proximal nesting populations were indistinguishable, suggesting recent colonization or less precise natal homing behaviour than documented for other marine turtle species. The evidence of natal homing, manifested on ecological time scales, may be erased in some populations by rapid rookery turnover resulting from climatic¯uctuation and the ephemeral nature of nesting habitat on a geological time scale. The evolutionary effective population size (N e ) is estimated from mtDNA data to be between 45 000 and 60 000, a value that exceeds current global census estimates of 26 000 to 43 000 adult females.

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Evolution of nuclear ribosomal DNA (rDNA) arrays of frogs of the genus Rana was examined among 32 species that last shared a common ancestor approximately 50 million years ago. Extensive variation in restriction sites exists within the transcribed and nontranscribed rDNA spacer regions among the species, whereas rDNA coding regions exhibit comparatively little interspecific variation in restriction sites. The most parsimonious phylogenetic hypothesis for the evolution of the group was constructed based on variation in restriction sites and internal spacer lengths among the 32 species of Rana and one species of Pyxicephalus (examined for outgroup comparison). This analysis suggests that R. sylvatica of North America is more closely related to the R. temporaria group of Eurasia than to other North American Rana. The hypothesized phylogeny also supports the monophyly of the R. boylii group, the R. catesbeiana group, the R. palmipes group, the R. tarahumarae group, and the R. pipiens complex. Furthermore, the restriction site data provide information about the evolution within and among these species groups. This demonstrates that restriction site mapping of rDNA arrays provides a useful molecular technique for the examination of historical evolutionary questions across considerable periods of time.

Southern Leaf Blight (SLB), Northern Leaf Blight (NLB), and Gray Leaf Spot (GLS) caused by Cochliobolus heterostrophus, Setosphaeria turcica, and Cercospora zeae-maydis respectively, are among the most important diseases of corn worldwide. Previously, moderately high and significantly positive genetic correlations between resistance levels to each of these diseases were identified in a panel of 253 diverse maize inbred lines. The goal of this study was to identify loci underlying disease resistance in some of the most multiple disease resistant (MDR) lines by the creation of chromosome segment substitution line (CSSL) populations in multiple disease susceptible (MDS) backgrounds. Four MDR lines (NC304, NC344, Ki3, NC262) were used as donor parents and two MDS lines (Oh7B, H100) were used as recurrent parents to produce eight BC3F4:5 CSSL populations comprising 1,611 lines in total. Each population was genotyped and assessed for each disease in replicated trials in two environments. Moderate to high heritabilities on an entry mean basis were observed (0.32 to 0.83). Several lines in each population were significantly more resistant than the MDS parental lines for each disease. Multiple quantitative trait loci (QTL) for disease resistance were detected for each disease in most of the populations. Seventeen QTL were associated with variation in resistance to more than one disease (SLB/NLB: 2; SLB/GLS: 7; NLB/GLS: 2 and 6 to all three diseases). For most populations and most disease combinations, significant correlations were observed between disease scores and also between marker effects for each disease. The number of lines that were resistant to more than one disease was significantly higher than would be expected by chance. Using the results from individual QTL analyses, a composite statistic based on Mahalanobis distance (Md) was used to identify joint marker associations with multiple diseases. Across all populations and diseases, 246 markers had significant Md values. However further analysis revealed that most of these associations were due to strong QTL effects on a single disease. Together, these findings reinforce our previous conclusions that loci associated with resistance to different diseases are clustered in the genome more often than would be expected by chance. Nevertheless true MDR loci which have significant effects on more than one disease are still much rarer than loci with single disease effects.

IgD has been suggested to be a recently developed Ig class, only present in rodents and primates. However, in this paper the cow, sheep, and pig Ig δ genes have been identified and shown to be transcriptionally active. The deduced amino acid sequences from their cDNAs show that artiodactyl IgD H chains are structurally similar to human IgD, where the cow, sheep, and pig IgD H chain constant regions all contain three domains and a hinge region, sharing homologies of 43.6, 44, and 46.8% with their human counterpart, respectively. According to a phylogenetic analysis, the Cδ gene appears to have been duplicated from the Cμ gene &gt;300 million yr ago. The ruminant μCH1 exon and its upstream region was again duplicated before the speciation of the cow and sheep, ∼20 million yr ago, inserted upstream of the δ gene hinge regions, and later modified by gene conversion. A short Sδ (switch δ) sequence resulting from the second duplication, is located immediately upstream of the bovine Cδ gene and directs regular μ-δ class switch recombination in the cow. The presence of Cδ genes in artiodactyls, possibly in most mammals, suggests that IgD may have some as yet unknown biological properties, distinct from those of IgM, conferring a survival advantage.