All the currently available data with regard to morphology, palaeontology, biochemical genetics, reproductive interactions and behaviour have been collated and analysed with combinations of phenetic and numerical phylogenetic methods, and integrated into a consensus evolutionary tree for European newts of the genus Triturus.

The oocytes of naturally occurring triploid females of the Ambystoma jeffersonianum complex each contain 84 lampbrush chromosomes. This constitutes hexaploidy (n = 14). The chromosomes are joined into pairs by chiasmata and form 42 bivalents. It is suggested that meiosis in triploid females is preceded by an endomitosis and the resulting sister chromosomes synapse to form pseudo-bivalents. Sperm from diploid males stimulate development of the triploid eggs but do not contribute chromosomes to the triploid nucleus. Bivalents in the oocytes of triploids have twice as many chiasmata as the corresponding bivalents in diploid animals. Such chiasmata cannot result in genetic recombination.

The arrangement of loops and chromomeres at the ends of lampbrush chromosomes in four species of bird is described with reference to chromomeres, loops and transcription units. Unlike the situation described in lampbrush chromosomes of amphibians, the lampbrush chromosomes of birds end in a terminal chromosome with conspicuous loops emerging from it. The fine-scale morphology of the ribonuclear protein matrix of these terminal loops is different from that of the majority of loops elsewhere on the chromosomes. In many cases the loops associated with the terminal chromomere are open ended, emerging from the chromomere but not returning to it at the other end. The distal ends of terminal open-ended loops therefore represent the true ends of the chromatids that make up a lampbrush half-bivalent. The pattern of binding of three telomeric DNA sequence probes to the terminal regions of bird lampbrush chromosomes, under conditions of DNA/DNA and DNA/RNA transcript in situ hybridization has been investigated by fluorescence in situ hybridization. All three probes gave the same results. With DNA/DNA and DNA/RNA transcript hybridization, three classes of structure were labelled: the terminal chromomere, a small number of interstitial chromomeres and the terminal transcription unit on telomere loops. Labelling of telomere loops, but not of terminal or interstitial chromomeres, was eliminated by ribonuclease treatment before in situ hybridization. The labelled regions of telomere loops were spaced away from the labelled terminal chromomere by an unlabelled sub telomeric transcription unit. After DNA/DNA in situ hybridization, no labelled loops were seen. DNA/RNA transcript in situ hybridization with single-stranded hexamers of each strand of telomeric DNA showed that the terminal transcription unit on telomere loops represents transcription exclusively from the C-rich strand of the repeat outwards towards the end of the chromosome. It is concluded that transcription specifically of the C-rich strand of strictly terminal clusters of telomere repeats is an obligatory event on the lampbrush chromosomes of birds and is unlikely to represent indiscriminate readthrough from proximally located gene elements.

The ZW bivalent has been identified and characterized in detail in its lampbrush form in oocytes of chicken, quail, turkey, pigeon, chaffinch and sparrow. The sex bivalent in all six species looks like a single highly asymmetrical chromosome. Most of it has the typical lampbrush organization. The terminal one-fifth is relatively thick and condensed and bears only a few pairs of lateral loops: this condensed terminal region is the W chromosome; the part with normal lampbrush morphology is the Z. The two are connected by a single near terminal chiasma. The fine scale morphology and arrangement of loops and markers on Z and W chromosomes are described for each species and lampbrush maps have been constructed. The identification of the lampbrush sex bivalent is based on the following criteria. The asymmetrical chromosome has two centromere regions. In the interstitial region of the asymmetrical chromosome where the junction between Z and W chromosomes is supposed to be, there are telomere-specific loops and telomeric DNA sequences and there is good morphological evidence for the presence of a chiasma. There are W chromosome specific DNA sequences in the region of the asymmetrical lampbrush chromosome that is thought to represent the W. Breed-specific variations in the morphology of the chicken W chromosome with respect to the sizes, numbers and arrangements of axial chromomeres and distributions of specified repeated DNA sequence families have been identified, offering one of the first examples of definitive correlation between a repeat family and a single chromomere. The lampbrush chromosomes of all the birds examined, except quail, terminate in distinctive free hanging loops. These are a novel feature in the sense that at the end of each chromatid there is a large transcription unit terminating in a cluster of telomeric DNA sequences.

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