Amphibians, which first adapted to terrestrial life in vertebrates, appeared around 350 million years ago (Mya) after the common ancestors of tetrapods (amphibians, reptiles, birds, and mammals) diverged from ray-finned fishes around 410 Mya, and are therefore important model animals for understanding vertebrate evolution. To date, there are many cytogenetic reports of amphibians. Moreover, recent improvements in techniques for cytogenetic and genomic analyses help accelerate the accumulation of the cytogenetic and genomic data from amphibians. Inferred from recent genomic and cytogenetic analyses using amphibians, I review karyotype and chromosome evolution, including sex chromosomes, polyploidy, and origins of microchromosomes, in not only amphibians but also entire vertebrates.

Meiosis is essential for gamete production, and involves two rounds of successive cell divisions, meiosis I and II. During prophase of meiosis I, chromosomes present meiosis-specific behavior, such as homologous chromosomes (HCs) pairing and synapsis. Here, we present the results found in the observation of meiosis in Veronica persica (2n = 28). The chromosome behavior of pollen mother cells (PMCs) at meiosis was evaluated via 4′,6-diamidino-2-phenylindole (DAPI)-staining and fluorescent in situ hybridization (FISH) using as the probes 5.8S rRNA and 25S rRNA-encoding sequences to detect 45S rDNA locus. DAPI staining confirmed the chromosome number and revealed conformational changes. The space occupied by the chromosomes within the PMCs seemed to shrink during late leptotene, which may assist HC pairing. 5.8S rDNA FISH signals were detected on seven bivalents, indicating that V. persica possesses seven 45S rDNA loci within its genome. Among the seven loci, some of them were associated with the nucleoli, but others were not. Taken together, this research has provided us new information concerning the overall and locus-specific dynamics of meiotic chromosomes in V. persica PMCs.

Somatic chromosomes in three subspecies of Chamaelirium hisauchianum from Japan were examined by fluorescent banding methods using probes for AT- or GC-rich DNA. Evidence showed that there are two kinds of constitutive heterochromatin; one constitutes portions deemed as centromeres rich in AT base pairs, and the other is of segments (probably of NORs) rich in GC base pairs. Nuclei at interphase had numerous scattered small centromeres each often accompanying a small condensed pericentric chromatin mass. Chromosomes at prophase displayed an irregular beaded structure, of which the knobby portions contained centromeres. Chromosomes at metaphase had approximately two to five pairs of sister centromeres aligning on the lateral sides and often protruded in opposite (polar) directions. Sister chromatids at anaphase separated parallel, turning their centromeres toward spindle poles. Chromosomes at telophase were unevenly unraveled, showing a configuration similar to those at prophase. Centromeres appeared compacted almost through the mitotic cell cycle. Data suggested that the chromosomes of C. hisauchianum comprise plural subunits each containing a centromere and are polycentric. It is evident that they contrastively differ from the monocentric, single-unitary chromosomes of the North American congener C. luteum reported previously.

Somatic chromosomes of Chamaelirium japonicum (2n=24) and C. koidzumianum (2n=24) from Japan were investigated by a fluorescent banding method using probes for AT- or GC-rich DNA. Data showed that there are two kinds of constitutive heterochromatin; one is rich in AT base pairs, constituting segments deemed as centromeres, and the other is rich in GC base pairs, constituting segments probably of NORs. The said centromeres were located on lateral sides of chromosomes at metaphase, varying in size from being almost invisible to ca. 0.3 μm in diameter. Discernibly distinct centromeres per nucleus counted ca. 12 on average in C. japonicum and ca. 25 in C. koidzumianum. Interphase chromosomes formed numerous small heterochromatic segments (HSs) that exceeded the somatic chromosome number ca. 3 times in C. japonicum and 4.3 times in C. koidzumianum. Each of the HSs contained a centromere, as long as the latter was discernible. It was suggested that every such HS contains a centromere, irrespective of the size and visibility of the latter, and that plural such segments organize a chromosome. The process of chromosomal diversification, especially of centromere organization, in Chamaelirium was inferred from a phylogenetic tree built from analyses of phenotypic characters.

Chromosome number, morphology and behavior in meiosis and mitosis were examined for three plants of Chamaelirium subsect. Chionographis. Somatic chromosomes of all the plants showed traits of holocentricity and were aberrant in constitution. One plant of C. japonicum subsp. japonicum was a triploid with 2n=36 (x=12), one plant deemed as a hybrid between C. japonicum subsp. yakusimense var. koshikiense and C. koidzumianum var. leiophyllum had 2n=25, including 1 fragmentary chromosome, and one plant identified as a hybrid between C. japonicum subsp. japonicum (2n=24) and C. hisauchianum subsp. minoense (2n=42) had 2n=33. The meiotic chromosomes of all these plants segregated equationally in the first division and reductionally in the second division. This is the first report of post reductional meiosis in Chamaelirium. The meiotic process in subsect. Chionographis was schematically illustrated and compared with that known for some other plants with holocentric chromosomes, including Luzula.

Rumex acetosa is a dioecious perennial and has sex chromosomes of XX in plants with pistillate flowers (female plants) and XYY in those with staminate flowers (male plants).

Normally, this species has six pairs of autosomes along with sex chromosomes. I collected one female plant with two heteromorphic chromosomes in somatic cells. The form of the two abnormal chromosomes suggested that translocation had occurred between them. To explore the chromosomal mutation of this plant, I examined the karyotype and meiotic chromosome behavior of heterozygote male offspring of this female plant. The translocation found in this study is considered as that from the long arm of the A5 autosome to the short arm of the A2 autosome and is considered a simple translocation (nonreciprocal translocation).