The chromosomal component of reproductive isolation in the grasshopper caledia captiva III. Chiasma distribution patterns in a new chromosomal taxon

Phylogenetic relationships among some Ateles species: the use of chromosomic and molecular characters

As with most platyrrhines, the systematics of Ateles is under discussion. In order to help clarify its systematic, we employed chromosomic and molecular characters to analyze the phylogenetic relationship among some species of the genus Ateles. Chromosomic studies were conducted on 14 atelid specimens: eight Ateles from A. paniscus, A. chamek, A. belzebuth and A. geoffroyi, and six Alouatta caraya. Ateles paniscus showed 2N=32, whereas A. chamek, A. belzebuth and A. geoffroyi presented 2N=34, XX/XY (with a submetacentric X and a variable Y) corroborated by male meiosis. Nucleotide sequence variation at the mitochondrial cytochrome c oxidase subunit II gene (COII) was analyzed in ten New World monkey specimens. Parsimony trees showed consistent phylogenetic relationships using both chromosomic forms and mitochondrial COII gene sequences as characters. Particularly, chromosomic phylogenies showed A. hybridus as a divergent taxon from the remaining group, whereas A. chamek, A. belzebuth and A. marginatus form an unresolved clade with A. geoffroyi as sister group.

Characterization of a hybrid zone between two chromosomal races of the weta Hemideina thoracica following a geologically recent volcanic eruption

Two chromosomal races (2n=17 and 2n=15; XO) of the weta Hemideina thoracica meet at the centre of a volcanic region in North Island, New Zealand. Five independent polymorphic genetic markers showed broadly coinciding, steep frequency clines from north to south across this zone beside the flooded crater, Lake Taupo. Three unlinked nuclear gene markers provide estimates of zone width that are at least twice the width of the chromosomal and mitochondrial clines, with cline centres displaced at least 2.5 km. The different zone widths and centres suggest that this hybrid zone is a semipermeable barrier reducing the introgression of the chromosomal markers more than genic markers. We estimate that this species of weta must have a dispersal rate of at least 100 m per generation using the time since the last Taupo eruption (1850 years ago), which covered an area of about 20 000 km2 with pyroclastic flow.

Staggered clines in a hybrid zone between two chromosome races of the harvestman Gagrellopsis nodulifera (Arachnida: Opiliones)

We analyzed a hybrid zone between two chromosome races (2n = 16 and 2n = 22) of a Japanese harvestman, Gagrellopsis nodulifera Sato and Suzuki (Arachnida: Opiliones: Phalangiidae). The hybrid zone is located in the eastern part of Tottori Prefecture, western Honshu. The width of the zone is approximately 5 to 15 km. Three independent tandem fusions/fissions seem to be the main cause of the karyotypic differences between the parental races. Ten karyotypic variants were found in the hybrid zone. They differed by numbers of diploid chromosomes and trivalents detected in meiosis. In most of the collecting sites, karyotypic heterozygotes were less common than expected. A positive correlation was found between number of trivalents in a karyotype and its deficiency rate. In some sites, the deficit of heterozygous individuals was accompanied by an excess of the intermediate homozygotes. One of the three transects across the zone was studied in detail. We found that three types of single heterozygotes (2n = 17, 2n = 19 and 2n = 21) formed a series of successive, spatially separated peaks along the transect. Two types of intermediate homozygotes (2n = 18 and 2n = 20) were also spatially separated. The most parsimonious explanation of such a structure is the staggering of clines of three tandem (or Robertsonian) fusion/fission variants that differentiate the parental races caused by selection against multiple heterozygotes. Analysis of nondisjunction in single heterozygotes demonstrated that there was a strong interindividual variation in nondisjunction rate. The mean frequency of aneuploid MII in single heterozygotes was 0.10 +/- 0.03. Crossover exchanges in some critical regions of trivalents result in abnormal chromosomal configurations: chromosomes with unequal chromatids and dicentric chromosomes. Frequency of crossover-induced chromosomal abnormalities was low in single heterozygotes (approximately equal to 4%), and was unexpectedly high in the double heterozygotes (approximately equal to 15%). Selection against karyotypic heterozygotes is considered as a main evolutionary force responsible for the structuring of the hybrid zone. A positive association between diploid chromosome number and altitude was found. The race 2n = 16 tended to occupy lower altitudes than the 2n = 22 parental race. Differences in ecological preferences may be a result of previous adaptations to different environments in allopatry. A hypothesis concerning the origin and evolution of the hybrid zone is proposed.

Genomic incompatibilities in the hybrid zone between house mice in Denmark: evidence from steep and non-coincident chromosomal clines for Robertsonian fusions

The pattern of chromosomal variation is investigated in house mice from the Danish hybrid zone between the translocation-prone Mus musculus domesticus and the chromosomally conservative M. m. musculus. The cytogenetic analysis confirmed the non-introgression of three pairs of Robertsonian (Rb) fusions from M. m. domesticus into the M. m. musculus genome. The geographic distribution of two of these Rb fusions was shown to follow staggered chromosomal clines which increased in steepness the closer they were to the centre of the hybrid zone as defined by allozymes. Analysis of alternate hypotheses suggests that chromosomal differentiation of the Danish domesticus occurred after contact was established with musculus. The staggering of the clines would reflect the order of arrival of the Rb fusions into the hybrid zone. Several models with different processes of underdominance of the chromosomal heterozygotes are discussed to account for the difference in width between clines. A selective model with increasing levels of genomic underdominance due to interaction with a progressively enriched musculus genome provides the best fit for the observed pattern. Selection against Rb fusions with little effect on the recombination of linked allozyme markers supports the view that no reduction in gene flow due to chromosomal heterozygosity is yet apparent through the hybrid zone and that only the centromeric segments of the Rb fusions are incompatible with the musculus genome.

Chromosomal rearrangements, speciation and the theoretical approach

Many theoretical papers investigating the relationship between chromosomal change and speciation are found to have been based on erroneous data. For rather than considering those negatively heterotic, or at least potentially negatively heterotic rearrangements which can have a possible role in speciation, these papers have included substantial amounts of information on rearrangements which are not implicated in this process. Common forms of chromosomal repatterning such as heterochromatic addition and polymorphism are in this category. Their inclusion in theoretical studies, often cited as supporting or opposing a chromosomal involvement in speciation, invalidates these findings. A new approach is suggested.