Variation within and between nucleolar organizer regions in Australian hylid frogs (Anura) shown by 18S+28S in-situ hybridization

Abstract

Five distinct classes of secondary constriction are found in the hylid frogs from the genera Litoria and Cyclorana, each of which is defined by its C-banding pattern and morphology (King, 1980, 1987). In-situ hybridization experiments utilizing 18S + 28S copy RNA probes derived from Xenopus and Drosophila rDNA templates, were made on nine species of frogs possessing the major constriction types. Types 1, 2, 4, and 5 are confirmed as being NORs. These results also indicate that type 1 and 2 constriction types are not differentially despiralized as previously suggested, but show absolute differences in the quantity of ribosomal DNA present. This variation took two forms, deletion polymorphism and amplification polymorphism. These differences were observed between homologues within cells and between cells within individuals. Animals possessing these 'despiralized' constrictions are therefore mosaics for both deletion and amplification polymorphisms. Polymorphism frequencies vary greatly between constriction types. Some specimens have a higher level of presence/absence heterozygosity, (L. moorei, type 2, L. nannotis type 5, L. raniformis (animal A, pair 8 type 2), than do others (L. peronii, L. rothii, L. caerulea). The above species also vary markedly in the degree and frequency of amplification of the NORs. The type 4 constrictions analysed (L. coplandi, L. lesueuri and C. novaehollandiae) have a particularly low frequency of presence/absence heterozygosity, and they have fewer size heteromorphisms between homologues. The type 3 ephemeral constrictions did not hybridize to cRNA probes at any stage. In all but one of the species studied, a single pair of chromosomes possessed an NOR. However, in L. raniformis these occurred on two pairs of chromosomes.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative cytogenetic studies of Bufo ictericus, B. paracnemis (Amphibia, Anura) and an intermediate form in sympatry

“…In general, these marks exhibited equivalent sizes in the same metaphase. In amphibians, especially in Anura, NORs tend to be coincident with positive C-band regions, and may present large C-band positive blocks associated with them (King, 1988(King, , 1990King et al, 1990). With regard to constitutional heterochromatic detection, the species B. ictericus and B. paracnemis and the intermediate form studied in this research were C-band positive in the centromeric region in all chromosomes.…”

Karyotype characterization, constitutive heterochromatin and nucleolus organizer regions of Paranaita opima (Coleoptera, Chrysomelidae, Alticinae)

“…These special types of heterochromatin flanking the NORs were not shown by the usual C-banding technique. The presence of constitutive heterochromatin flanking or inserting NORs has been found in the chromosomes of several different groups of animals (Cabrero et al, 1986;King et al, 1990;Pendás et al, 1993;Silva et al, 2000;Vicari et al, 2003Vicari et al, , 2005. The presence of this NOR-associated heterochromatin can restrict genetic recombination in the adjacent region, avoiding the formation of chiasmata or inducing their formation in other chromosome regions (John and King, 1982King, , 1985 and also can alter the genetic expression of the NORs (Arnold and Shaw, 1985) or can represent breakpoints that facilitate the dispersion of the NORs (Moreira-Filho et al, 1984;Reed Phillips, 1995).…”

Chromosomal characterization of Pseudonannolene strinatii (Spirostreptida, Pseudonannolenidae)

“…According to the authors, this difference might be attributed to two factors: differential genetic activity of the rDNA segments and duplication of the amount of rDNA. KING et al (1990) state that the high index of heteromorphisms of the NORs suggests that a significant incidence of non-reciprocal exchange might be occurring among homologues. Unfortunately the lack of deepened studies in cytogenetics of millipedes give us few data to make comparison inside the group, so maybe in the NOR heteromorphism of P. strinatii all of these events could be involved.…”