Peter Luykx

Distribution of Karyotypes of the Cryptocercus punctulatus Species Complex (Dictyoptera: Cryptocercidae) in the Southern Appalachians: Relation to Habitat and History

Abstract The distributional pattern of the four known karyotypes (male 2n = 37, 39, 43, 45) of the Cryptocercus punctulatus Scudder species complex is reported, based on 71 sites in the Southern Appalachian Mountains with an emphasis on western North Carolina. Populations with different karyotypes are geographically structured in a mosaic, with at least one karyotype occurring in two disjunct regions. Abrupt geographic transitions between karyotypes suggest a parapatric distribution. We found no overlap in the distribution of the different karyotypes, as recently suggested. Although the boundary zones between karyotypes do not appear to coincide with physical or ecological barriers to dispersal, several transitions between karyotypes occur on or near the highest mountains in the southern Appalachians. We suggest that the different karyotypes arose by vicariance, with current boundaries formed by secondary contact when populations isolated in glacial refugia subsequently spread into high-mountain habitats. Because of their dependence on mature mesic forests, populations of the cockroach likely advance up and down mountainsides in cycles of advances and retreats dictated by climatic oscillations that raise and lower the timberline. We discuss the taxonomic status of the different karyotype groups in the C. punctulatus complex, and conclude that more exacting evidence is required to establish if species-level status is warranted. The conclusions of certain earlier studies are weak because, among other things, karyology was not examined in the sampled specimens, including those designated as types.

Structure and behavior of contractile vacuoles in Chlamydomonas reinhardtii

SummaryThe contractile vacuole (CV) cycle ofChlamydomonas reinhardtii has been investigated by videomicroscopy and electron microscopy. Correlation of the two kinds of observation indicates that the total cycle (15 s under the hypo-osmotic conditions used for videomicroscopy) can be divided into early, middle, and late stages. In the early stage (early diastole, about 3 s long) numerous small vesicles about 70–120 nm in diameter are present. In the middle stage (mid-diastole, about 6 s long), the vesicles appear to fuse with one another to form the contractile vacuole proper. In the late stage (late diastole, also about 6 s long), the CV increases in diameter by the continued fusion of small vesicles with the vacuole, and makes contact with the plasma membrane. The CV then rapidly decreases in size (systole, about 0.2 s). In isosmotic media, CVs do not appear to be functioning; under these conditions, the CV regions contain numerous small vesicles typical of the earliest stage of diastole. Fine structure observations have provided no evidence for a two-component CV system such as has been observed in some other cell types. Electron microscopy of cryofixed and freeze-substituted cells suggests that the irregularity of the profiles of larger vesicles and vacuoles and some other morphological details seen in conventionally fixed cells may be shrinkage artefacts. This study thus defines some of the membrane events in the normal contractile vacuole cycle ofChlamydomonas, and provides a morphological and temporal basis for the study of membrane fusion and fluid transport across membranes in a cell favorable for genetic analysis.

Geographic variation of sex-linked translocation heterozygosity in the termite Kalotermes approximatus snyder (Insecta: Isoptera)

The primitive termite Kalotermes approximatus carries a number of reciprocal translocations (segmental interchanges) that are linked to the sex-determining mechanism in such a way that males are permanent structural heterozygotes, forming long chains or rings of chromosomes in meiosis, while females are structural homozygotes, forming only bivalents. A survey of male meiosis from collections covering nearly the whole species range in the southeastern United States reveals considerable variation in the number of translocations: males with a diploid number of 32 or 33 have meiotic chains of 11, 13, 14, 15, 16, and 17 or 19 chromosomes. The different types can be arranged in an evolutionary series of rearrangements involving translocations or Robertsonian fusions between chromosomal elements in the ring and those outside. In addition, the existence of a closed chain (ring) of 16, and of four different types of chain of 13, indicate that similar rearrangements have occurred among chain elements. The geographic pattern of these rearrangements suggests that their selection accompanied the expansion of the species northward from southern Florida sometime since the last glaciation or, alternatively, that as they arose the new translocation types successively supplanted the ancestral types, preferentially in the east-central portion of the range.

Holozygosity for sex-linked genes in males of the termiteIncisitermes schwarzi

The termiteIncisitermes schwarzi has multiple sex chromosomes that have arisen by repeated translocations between autosomes and previously existing sex chromosomes. Two sex-linked allozyme loci—Acp-1 andEst-3—are holozygous, not hemizygous, in males (the heterogametic sex). Both loci show less than 1% crossing-over between X and Y chromosomes, and alleles of both are in marked disequilibrium with respect to X vs Y linkage. The two loci assort independently in female meiosis, indicating that they lie on different sex chromosomes. But they are tightly linked in male meiosis because of nonrandom assortment of the multiple X and Y chromosomes in males of this species. The findings of holozygosity and strong linkage disequilibrium suggest that differential selection in the two sexes at or near these loci may be responsible for the establishment of the translocations in this species. The existence of active Y-linked alleles also suggests that the translocations may have occurred recently.

The Cryptocercus punctulatus species complex (Dictyoptera: Cryptocercidae) in the eastern United States: comparison of cuticular hydrocarbons, chromosome number, and DNA sequences.

The goal of the current study was to determine if cuticular hydrocarbons could be used to empirically delimit taxa within the Cryptocercus punctulatus species complex in the eastern United States. Cockroaches were collected from rotting logs in 22 locations across four states. Hydrocarbon phenotypes and two mitochondrial (16S and COII) genes and one nuclear (ITS2) gene were independently analyzed to determine their relationship with chromosome number. Five distinct hydrocarbon phenotypes were found, but these were only partly congruent with chromosome number and thus with purported species descriptions. Molecular and cuticular hydrocarbon data each indicate that Cryptocercus with a male karyotype of 2n=43 belong to at least two distinct, distantly related lineages. One 2n=43 lineage is sister group to the 2n=37 and 2n=39 clade, and has a unique hydrocarbon profile. The other 2n=43 lineage is sister group to the 2n=45 samples, and its cuticular hydrocarbons group with four samples of the 2n=45 lineage. The cuticular hydrocarbons of two additional 2n=45 samples diverge from this assemblage. Results indicate cuticular hydrocarbons and chromosome number have some degree of evolutionary independence; neither is completely reliable in delineating historical lineages. Our data provide support for the parallel model of chromosome evolution in the species complex.

Osmoregulatory mutants that affect the function of the contractile vacuole inChlamydomonas reinhardtii

SummaryFour independent osmoregulatory mutants,osml, osm3,osm4, and osm7, were isolated on the basis of their requirement for growth medium of high osmotic strength. In normal low-osmoticstrength medium, in contrast to wild-type cells, the mutants grow poorly or not at all; in distilled water mutant cells are immobilized and eventually swell and burst. The mutants were examined by ordinary brightfield and phase-contrast microscopy, videomicroscopy, and electron microscopy. The four mutants showed different defects in the contractile vacuole (CV) cycle. Timing of various stages of the CV cycle showed thatosm1 was affected primarily in the early stage of the cycle when the CV begins to grow,osm3 primarily in midcycle when vacuoles fuse to form the CV proper,osm7 at a late stage of the cycle at docking and fusion of the CV with the plasma membrane, andosm4 during contraction of the CV. At the electron microscopic level, in dilute medium, mutant cells by comparison with wild-type cells had large autophagosomes, swollen mitochondria, and dilated ER cisternae. Although electron microscopy showed general abnormalities of the contractile vacuoles consistent with the videomicroscopic observations of living cells, no obvious vacuole membrane abnormalities were seen which would explain the mutational defects. The mutations help define the separate processes that contribute to the coordinated CV cycle inChlamydomonas, and open the way to eventual isolation of some of the genes responsible for CV function.

Genetic Relations Among Castes in Lower Termites

Publisher Summary The chapter focuses on genetic relations among castes in lower termites. Genetic relatedness among members of colonies of lower termites is influenced by the presence of multiple sex chromosomes, the sex of the individuals, the sex-ratio of the colony as a whole, the existence of replacement reproductives, and by the general level of inbreeding in the population. Studies on I. schwarzi show that about half the diploid set of 32 chromosomes behave like sex chromosomes, that the mean male–female sex ratio in natural populations is 1.24, and that the relatively low levels of inbreeding observed in natural populations are accounted for by the presence of replacement reproductives in about 25% of the colonies. Interactions between colony members of the same sex are more frequent or more significant than between individuals of the opposite sex. The evolution of soldier altruism by kin-selection in termites remains a theoretical possibility.

A sex-linked esterase locus and translocation heterozygosity in a termite

SummarySlow vs. fast alleles at an esterase locus show a sex-linked pattern of inheritance in colonies of the termite Incisitermes schwarzi. As might be expected where a gene has only recently been brought into sex-linkage by means of a translocation between an autosome and a sex-chromosome, males as well as females can be heterozygous; males are therefore “holozygous”, not hemizygous, for this gene. The rate of exchange of this gene between X and Y chromosomes is 2/266 or less. In the founding reproductives (male or female), X chromosomes may have either the fast or the slow allele; in males, only Y chromosomes carrying the fast allele have been found. This linkage disequilibrium may be due to a founder effect, to “hitch-hiking” of the gene with the translocation, or to a sex difference in the fitness of gene combinations at or near this locus.

Molecular phylogeny of Cryptocercus wood-roaches based on mitochondrial COII and 16S sequences, and chromosome numbers in Palearctic representatives

Abstract Woodroaches of the genus Cryptocercus are subsocial and xylophagous cockroaches, distributed in North America and Asia. Studies on male chromosome number in Nearctic species have shown that diploid numbers vary from 2n=37 to 2n=47; numbers from Palearctic species were heretofore unknown. Two hypotheses have been proposed to explain the varying number of chromosomes among Nearctic species: the serial reduction hypothesis, and the parallel scenario. We performed phylogenetic analyses of the COII gene in these species and found evidence for the topology (47(45(43(39,37), which is congruent with the serial reduction hypothesis. We also determined chromosome numbers for the first time in Palearctic species, and found Cryptocercus primarius and Cryptocercus relictus to have relatively low chromosome numbers (2n=17–21) compared to their Nearctic relatives. Finally, our study determined the phylogenetic position of Cryptocercus primarius among other Asian taxa.

NXSensor web tool for evaluating DNA for nucleosome exclusion sequences and accessibility to binding factors

Nucleosomes, a basic structural unit of eukaryotic chromatin, play a significant role in regulating gene expression. We have developed a web tool based on DNA sequences known from empirical and theoretical studies to influence DNA bending and flexibility, and to exclude nucleosomes. NXSensor (available at ) finds nucleosome exclusion sequences, evaluates their length and spacing, and computes an ‘accessibility score’ giving the proportion of base pairs likely to be nucleosome-free. Application of NXSensor to the promoter regions of housekeeping (HK) genes and those of tissue-specific (TS) genes revealed a significant difference between the two classes of gene, the former being significantly more open, on average, particularly near transcription start sites (TSSs). NXSensor should be a useful tool in assessing the likelihood of nucleosome formation in regions involved in gene regulation and other aspects of chromatin function.