Organization and molecular cytogenetics of a satellite DNA family from Hoplias malabaricus (Pisces, Erythrinidae)

The chromosomes of the primitive South American teleost fish Hoplias malabaricus have been analyzed by classical cytogenetic (C-, AgNOR-, Hoechst 33258-, and Q-banding) techniques. A highly repetitive DNA family has been cloned and sequenced. It is a tandemly repeated sequence of about 355 bp, yielding an overall base pair composition of 67% AT with long runs of > 50% As and 70% Ts. Analysis of sequence variation has allowed the further categorization of Hoplias satellite DNA into two evolutionary related subfamilies A and B, distinguishable by characteristic insertions and deletions within this 355-bp monomer. Subfamily A satellite is found (in diverged form) at the centromeres of most H. malabaricus chromosomes. Sequence variants are clustered on specific chromosomal subsets. Subfamily B satellite is highly specific for the paracentromeric heterochromatin on one particular chromosome pair by fluorescence in situ hybridization. These results indicate that the Hoplias satellite DNA family has evolved in a concerted manner predominantly via recombination events involving homologous, rather than non-homologous chromosome regions. The clones isolated here may be useful for the molecular, genetic, and cytological analysis of the genus Hoplias.

Chromosome banding in Amphibia. XIX. Primitive ZW/ZZ sex chromosomes in Buergeria buergeri (Anura, Rhacophoridae)

The karyotype of the common bell-ring frog, Buergeria buergeri, is characterized by primitive ZW/ZZ (female/male) sex chromosomes. The only difference between the Z and W is the presence of a nucleolus organizer region (NOR) and its associated constitutive heterochromatin in the Z. This causes a sex-specific difference in the number of ribosomal RNA genes, with a male:female ratio of about 2:1. In the cell nuclei of the various tissues analyzed the NORs on both Z chromosomes are transcriptionally active. During meiosis in females, the ZW chromosomes form a characteristic lampbrush sex bivalent with a conspicuous absence of chiasmata along half of their long arms. No further morphological differences are present in these primitive sex chromosomes. The results indicate that there is no Z-chromosome inactivation mechanism operating in this amphibian. Experimental sex reversal of genetic (ZW) females to functional males was achieved by estrogen treatment of tadpoles. Mating of sex-reversed females with normal females yields the expected 25% lethal nucleoli-less WW embryos, which die at the tail-bud stage. The sex of the remaining viable embryos with one or two nucleoli determined after metamorphosis was in a female:male ratio of 1.57:1. These genetic experiments confirm that a primitive ZW/ZZ (female/male) type of chromosomal sex determination operates in this species.

Chromosome banding in Amphibia. XVIII. Karyotype evolution and genomic size variation in Pleurodema (Anura, Leptodactylidae)

DNA flow cytophotometric measurements demonstrate that the quantity of nuclear DNA of the South American leptodactylid frog Pleurodema brachyops is 3.4 times greater than that of P. thaul. Nevertheless, the conventionally stained karyotypes of both species are nearly identical. In the metaphase chromosomes of P. brachyops, the chromatin has a distinctly higher degree of packaging than in those of P. thaul. C-banding reveals that almost 6 times the constitutive heterochromatin is present in the karyotype of P. brachyops than in the karyotype of P. thaul. Analysis of fluorescence banding patterns shows that the chromosomes of P. brachyops contain AT- and GC-rich heterochromatin, whereas the karyotype of P. thaul is devoid of brightly fluorescing heterochromatin. The substantial differences in the genome sizes of Pleurodema is explained by homogeneous, symmetrical changes in the amounts of all DNA sequence classes along all chromosomes, which preserved the ancestral morphology of the chromosomes.

Chromosome banding in Amphibia. XVII. First demonstration of multiple sex chromosomes in amphibians: Eleutherodactylus maussi (Anura, leptodactylidae)

A cytogenetic study performed on a population of the South American leptodactylid frog Eleutherodactylus maussi revealed multiple sex chromosomes of the X1X1X2X2 female/X1X2Y male (= XXAA female/XXAY male) type. The diploid chromosome number is 2n = 36 in all females and 2n = 35 in most males. The multiple sex chromosomes originated by a centric fusion between the original Y chromosome and a large autosome. In male meiosis the X1X2Y (= XXAY) multiple sex chromosomes form a classical trivalent configuration. E. maussi is the first species discovered in the class Amphibia that is distinguished by a system of multiple sex chromosomes. Only one single male was found in the population with 2n = 36 chromosomes and lacking the Y-autosomal fusion. This karyotype (XYAA male) is interpreted as the ancestral condition, preceding the occurrence of the Y-autosome fusion.

Chromosome banding in Amphibia. XVI. High-resolution replication banding patterns in Xenopus laevis

High-resolution replication banding patterns were induced in prometaphase and prophase chromosomes of Xenopus laevis by treating kidney cell lines with 5-bromodeoxyuridine (BrdU) and deoxythymidine (dT) in succession. Up to 650 early and late replicating bands per haploid karyotype were demonstrated in the very long prophase chromosomes. This permits an exact identification of all chromosome pairs of X. laevis. Late replicating heterochromatin was located by analysing the time sequence of replication throughout the second half of S-phase. Neither heteromorphic sex chromosomes nor sex chromosome-specific replication bands were demonstrated in the heterogametic ZW females of X. laevis. A detailed examination of the BrdU/dT-labelled prometaphases and prophases revealed that the X. laevis chromosomes can be arranged in groups of four (quartets), most of which show conspicuous similarities in length, centromere position, and replication pattern. This is interpreted as further evidence for an ancient allotetraploid origin of X. laevis.

Nucleolar transcriptional activity in mouse Sertoli cells is dependent on centromere arrangement

Experimental evidence suggests that centromere arrangement is relevant to the expression of ribosomal genes in murine Sertoli cells. Nuclei endowed with a nucleolus inactive in rRNA synthesis presented several clusters, each containing a bunch of individual centromeres. RNA polymerase I was not cytochemically detected in the nucleolar structure, which contained only small amounts of fibrillarin. In the course of nucleolar activation, the centromeres within the separate clusters became fused into larger centromeric bodies. Synthesis of precursor rRNAs and their processing were visualized by strong nucleolar fluorescence signals using antibodies to RNA polymerase I and fibrillarin.

On the highest chromosome number in mammals

The mitotic and meiotic chromosomes of the semiaquatic rodent Ichthyomys pittieri (Rodentia, Cricetinae) from Venezuela were analyzed by means of conventional staining and several banding techniques. The diploid chromosome number of this rare species is 2n = 92, which is the highest value known for mammals. It is assumed that this exceptionally high chromosome number is the result of repeated centric fissions. The karyotype of I. pittieri was compared with that of Anotomys leander, for which a diploid number of 2n = 92 has also been reported. The karyological relationships existing within the Neotropical Cricetidae are summarized.