Academic literature on the topic 'Chromosomes, speciation, spider'

Chiasma position is compared among huntsman spider populations possessing telocentric chromosomes and Robertsonian metacentrics in the homozygous and heterozygous condition. The results indicate that heterozygosity for fusions results in a significant difference in chiasma position from both the telocentric forms and fusion homozygotes. Differences between fusion homozygotes and ancestral telocentric forms are small or absent, and there is no evidence of chiasma interference across the centromere. It is argued that the observed association between Robertsonian rearrangements and speciation may be as much a result of chiasma redistribution as of segregational factors.Key words: spiders, chiasma position, recombination, speciation.

The genus Dysdera Latreille, 1804, a species-rich group of spiders that includes specialised predators of woodlice, contains several complexes of morphologically similar sibling species. Here we investigate species limits in the D. erythrina (Walckenaer, 1802) complex by integrating phenotypic, cytogenetic and molecular data, and use this information to gain further knowledge on its origin and evolution. We describe 16 new species and redescribe four poorly known species belonging to this clade. The distribution of most of the species in the complex is limited to southern France and the north-eastern Iberian Peninsula. The species studied do not show any obvious differences in habitat preference, and some of them even occur sympatrically at certain sites. They probably feed on the same type of prey as they readily capture woodlice. On the other hand, they differ in body size, mouthparts shape, sculpturing of carapace, morphology of the copulatory organs, karyotype and DNA sequences. Experimental interspecific mating showed a partial precopulatory behavioural barrier between D. erythrina and D. cechica, sp. nov. Our data suggest that karyotype evolution of the complex included chromosome fusions and fissions as well as translocations (between autosomes as well as autosomes and sex chromosomes). We hypothesise that chromosome rearrangements generating reproductive incompatibility played a primary role in speciation within Dysdera complexes. Dysdera spiders are poor dispersers, and their original distribution areas (forested areas in the Mediterranean) were repeatedly fragmented during Quarternary climatic oscillations, facilitating integration of chromosome rearrangements into karyotypes by genetic drift. Sympatric occurrence of closely related species may have been promoted by prey segregation as suggested by differentiation in body size in co-occurring species. The following new species are described: D. catalonica, sp. nov., D. cechica, D. dolanskyi, sp. nov., D. fabrorum, sp. nov., D. garrafensis, sp. nov., D. graia, sp. nov., D. kropfi, sp. nov., D. minairo, sp. nov., D. portsensis, sp. nov., D. pradesensis, sp. nov., D. pyrenaica, sp. nov., D. quindecima, sp. nov., D. septima, sp. nov., D. stahlavskyi, sp. nov., D. tredecima, sp. nov. and D. undecima, sp. nov.

Chromosomal evolution has long been linked with the process of organismal speciation, and many different theories have been suggested over the years to explain why this would be so. These theories can be loosely grouped into two eras. Classical chromosomal speciation models focused on negative heterosis of chromosomal rearrangements causing malsegregation and germ cell death in hybrids. More recent models examine the effects of reduced recombination around rearrangements and the impact this can have on sequence evolution, specifically the accumulation of genetic incompatibilities.¶ The huntsman spider Delena cancerides is known to be highly chromosomally variable, and to have reduced recombination near fusions. However, this species has previously only been interpreted with reference to the classical models of chromosomal speciation, the expectations of which it does not fit well. Broad-scale sampling of this spider has revealed extensive chromosomal diversity and complexity. Twenty one chromosomally differentiated populations (karyomorphs) of this spider have now been described, including those with the putatively ancestral configuration of all telocentric bivalents at meiosis (tII), and many that are saturated for Robertsonian fusions. These include up to six different karyomorphs with metacentric bivalents (mII), eight karyomorphs that form a chain of chromosomes at male meiosis, and six karyomorphs that form two separate but co-segregating chains.¶ A computer simulation was used to test hypotheses regarding the evolution of this chromosomal diversity, which indicated that fusions are likely to have accumulated gradually, possibly due to meiotic drive. Historical phylogeographic analyses have shown that deep cryptic divisions exist which are concordant with the chromosomal diversity.¶ Hybridization experiments have suggested that many hybrid zones between karyomorphs of this species are tension zones, and that genetic incompatibilities are likely to play an important role in generating partial reproductive isolation of karyomorphs. Furthermore, several hybrid zones appear to have been modified by staggered clines. The staggering of clines is thought to ameliorate reproductive isolation mechanisms that are dependent on epistatic fitness interactions, and so may prevent diverging populations progressing towards speciation. ¶Therefore, on the basis of the available evidence, D. cancerides may fit the recombination suppression model of chromosomal speciation, although it may be unlikely that the karyomorphs will progress towards full species status. Hence, this species may in the future make a highly informative model organism for investigating the early stages of genetic reproductive isolation associated with chromosomal rearrangements.

The huntsman spider Delena cancerides is known to be highly chromosomally variable, and to have reduced recombination near fusions. However, this species has previously only been interpreted with reference to the classical models of chromosomal speciation, the expectations of which it does not fit well. Broad-scale sampling of this spider has revealed extensive chromosomal diversity and complexity. Twenty one chromosomally differentiated populations (karyomorphs) of this spider have now been described, including those with the putatively ancestral configuration of all telocentric bivalents at meiosis (tII), and many that are saturated for Robertsonian fusions. These include up to six different karyomorphs with metacentric bivalents (mII), eight karyomorphs that form a chain of chromosomes at male meiosis, and six karyomorphs that form two separate but co-segregating chains.¶ A computer simulation was used to test hypotheses regarding the evolution of this chromosomal diversity, which indicated that fusions are likely to have accumulated gradually, possibly due to meiotic drive. Historical phylogeographic analyses have shown that deep cryptic divisions exist which are concordant with the chromosomal diversity.¶ Hybridization experiments have suggested that many hybrid zones between karyomorphs of this species are tension zones, and that genetic incompatibilities are likely to play an important role in generating partial reproductive isolation of karyomorphs. Furthermore, several hybrid zones appear to have been modified by staggered clines. The staggering of clines is thought to ameliorate reproductive isolation mechanisms that are dependent on epistatic fitness interactions, and so may prevent diverging populations progressing towards speciation. ¶Therefore, on the basis of the available evidence, D. cancerides may fit the recombination suppression model of chromosomal speciation, although it may be unlikely that the karyomorphs will progress towards full species status. Hence, this species may in the future make a highly informative model organism for investigating the early stages of genetic reproductive isolation associated with chromosomal rearrangements.

Orb-weavers (Araneidae) are a diversified spider family comprising more than 3,100 species in more than 170 genera. Together with 13 other families, they con- stitute to superfamily Araneoidea. The presented thesis focuses on karyotype evo- lution of Araneidae, including its comparison with a related family Tetragnathidae. The results obtained from 19 araneid and four tetragnathid species confirm previ- ously postulated hypothesis that the ancestral karyotype of Araneoidea (including Araneidae) consists of 24 acrocentric chromosomes in males, including two acro- centric X chromosomes of system X1X20. However, there is a tendency of 2n decrease in some araneids due to centric fusions. In these cases, centric fusions affected most autosomes (and sometimes gonosomes as well); number of chromosome pairs de- creased from 11 to six. Three independent reduction events were detected in this thesis. Furthermore, pattern of nucleolar organizer regions (NORs) was studied in this thesis using fluorescent in situ hybridization, since data on evolution of this marker are scarce in spiders. Striking variability in NORs number was discovered, ranging from one to 13 loci. Remarkably, multiple centric fusions were always ac- companied by considerable increase of NORs number. In araneids and tetragnathids possessing...