Character displacement in some Cnemidophorus lizards revisited: A phylogenetic analysis

Ecological studies have demonstrated the role of competition in structuring communities; however, the importance of competition as a vehicle for evolution by natural selection and speciation remains unresolved. Study systems of insular faunas have provided several well known cases where ecological character displacement, coevolution of competitors leading to increased morphological separation, is thought to have occurred (e.g., anoline lizards and geospizine finches). Whiptail lizards (genus Cnemidophorus) from the islands of the Sea of Cortez and the surrounding mainland demonstrate a biogeographic pattern of morphological variation suggestive of character displacement. Two species of Cnemidophorus occur on the Baja peninsula, one relatively large (Cnemidophorus tigris) and one smaller (Cnemidophorus hyperythrus). Oceanic islands in the Sea of Cortez contain only single species, five of six having sizes intermediate to both species found on the Baja peninsula. On mainland Mexico C. hyperythrus is absent, whereas C. tigris is the smaller species in whiptail guilds. Here we construct a phylogeny using nucleotide sequences of the cytochrome b gene to infer the evolutionary history of body size change and historical patterns of colonization in the Cnemidophorus system. The phylogenetic analysis indicates that (i) oceanic islands have been founded at least five times from mainland sources by relatives of either C. tigris or C. hyperythrus, (ii) there have been two separate instances of character relaxation on oceanic islands for C. tigris, and (iii) there has been colonization of the oceanic island Cerralvo with retention of ancestral size for Cnemidophorus ceralbensis, a relative of C. hyperythrus. Finally, the phylogenetic analysis reveals potential cryptic species within mainland populations of C. tigris.

Complete sequencing of the Fugu WAGR region from WT1 to PAX6: Dramatic compaction and conservation of synteny with human chromosome 11p13

The pufferfish Fugu rubripes has a genome ≈7.5 times smaller than that of mammals but with a similar number of genes. Although conserved synteny has been demonstrated between pufferfish and mammals across some regions of the genome, there is some controversy as to what extent Fugu will be a useful model for the human genome, e.g., [Gilley, J., Armes, N. & Fried, M. (1997) Nature (London) 385, 305–306]. We report extensive conservation of synteny between a 1.5-Mb region of human chromosome 11 and <100 kb of the Fugu genome in three overlapping cosmids. Our findings support the idea that the majority of DNA in the region of human chromosome 11p13 is intergenic. Comparative analysis of three unrelated genes with quite different roles, WT1, RCN1, and PAX6, has revealed differences in their structural evolution. Whereas the human WT1 gene can generate 16 protein isoforms via a combination of alternative splicing, RNA editing, and alternative start site usage, our data predict that Fugu WT1 is capable of generating only two isoforms. This raises the question of the extent to which the evolution of WT1 isoforms is related to the evolution of the mammalian genitourinary system. In addition, this region of the Fugu genome shows a much greater overall compaction than usual but with significant noncoding homology observed at the PAX6 locus, implying that comparative genomics has identified regulatory elements associated with this gene.

Farnesyltransferase inhibitors induce dramatic morphological changes of KNRK cells that are blocked by microtubule interfering agents

Farnesyltransferase inhibitors (FTIs) exhibit the remarkable ability to inhibit transformed phenotypes of a variety of human cancer cell lines and to block the growth of cancer cells in a number of animal model systems. In this paper, we report that the addition of FTI to v-K-ras- transformed NRK cells (KNRK) results in dramatic morphological changes. Within 24 h after the addition of FTI, the round morphology of KNRK cells was changed to an elongated (flattened and spread out) morphology resembling those of untransformed NRK cells. No morphological effects were seen when similar concentrations of FTI were added to NRK cells. Phalloidin staining showed that FTI treatment did not restore the disrupted actin cytoskeleton in KNRK cells. In contrast, FTI addition resulted in the appearance of extensive microtubule networks in KNRK cells. The addition of a low concentration (1.2 nM) of vincristine or vinblastine, agents that interfere with microtubule dynamics, blocked the FTI-induced morphological changes in KNRK cells. In contrast, cytochalasin B, which interferes with actin polymerization, did not block the morphological changes. The FTI-induced morphological changes were associated with a decrease in the percentage of cells in S-phase, and the addition of 1.2 nM vincristine did not have additional effects on cell cycle progression. A higher concentration (12 nM) of vincristine caused synergistic effect with FTI to enrich dramatically KNRK cells in G2/M phase. These results suggest that FTI affects cell morphology and that microtubule dynamics are involved in these processes.

Remodeling of the postnatal mouse testis is accompanied by dramatic changes in stem cell number and niche accessibility

Little is known about stem cell biology or the specialized environments or niches believed to control stem cell renewal and differentiation in self-renewing tissues of the body. Functional assays for stem cells are available only for hematopoiesis and spermatogenesis, and the microenvironment, or niche, for hematopoiesis is relatively inaccessible, making it difficult to analyze donor stem cell colonization events in recipients. In contrast, the recently developed spermatogonial stem cell assay system allows quantitation of individual colonization events, facilitating studies of stem cells and their associated microenvironment. By using this assay system, we found a 39-fold increase in male germ-line stem cells during development from birth to adult in the mouse. However, colony size or area of spermatogenesis generated by neonate and adult stem cells, 2–3 months after transplantation into adult tubules, was similar (∼0.5 mm2). In contrast, the microenvironment in the immature pup testis was 9.4 times better than adult testis in allowing colonization events, and the area colonized per donor stem cell, whether from adult or pup, was about 4.0 times larger in recipient pups than adults. These factors facilitated the restoration of fertility by donor stem cells transplanted to infertile pups. Thus, our results demonstrate that stem cells and their niches undergo dramatic changes in the postnatal testis, and the microenvironment of the pup testis provides a more hospitable environment for transplantation of male germ-line stem cells.

Negative frequency-dependent selection maintains a dramatic flower color polymorphism in the rewardless orchid Dactylorhiza sambucina (L.) Soò

The orchid Dactylorhiza sambucina shows a stable and dramatic flower-color polymorphism, with both yellow- and purple-flowered individuals present in natural populations throughout the range of the species in Europe. The evolutionary significance of flower-color polymorphisms found in many rewardless orchid species has been discussed at length, but the mechanisms responsible for their maintenance remain unclear. Laboratory experiments have suggested that behavioral responses by pollinators to lack of reward availability might result in a reproductive advantage for rare-color morphs. Consequently, we performed an experiment varying the relative frequency of the two color morphs of D. sambucina to test whether rare morph advantage acted in the natural habitat of the species. We show here clear evidence from this manipulative experiment that rare-color morphs have reproductive advantage through male and female components. This is the first demonstration, to our knowledge, that negative frequency-dependent selection through pollinator preference for rare morphs can cause the maintenance of a flower-color polymorphism.

Sexually antagonistic coevolution of a postmating-prezygotic reproductive character in desert Drosophila

Rapid divergence in postmating-prezygotic characters suggests that selection may be responsible for generating reproductive barriers between closely related species. Theoretical models indicate that this rapid divergence could be generated by a series of male adaptations and female counteradaptations by means of sexual selection or conflict, but empirical tests of particular mechanisms are generally lacking. Moreover, although a male–female genotypic interaction in mediating sperm competition attests to an active role of females, molecular or morphological evidence of the female's participation in the coevolutionary process is critically needed. Here we show that postmating-prezygotic variation among populations of cactophilic desert Drosophila reflects divergent coevolutionary trajectories between the sexes. We explicitly test the female's role in intersexual interactions by quantifying differences in a specific postmating-prezygotic reproductive character, the insemination reaction mass, in two species, Drosophila mojavensis and Drosophila arizonae. A series of interpopulation crosses confirmed that population divergence was propelled by male–female interactions, a prerequisite if the selective forces derive from sexual conflicts. An association between the reaction mass and remating and oviposition behavior argues that divergence has been propelled by sexually antagonistic coevolution, and potentially has important implications for speciation.