Evidence for a sex-segregated migration in the humpback whale (Megaptera novaeangliae).

Existing population models for humpback whales assume that all individuals within a population undertake the annual migration from feeding areas in high latitudes to breeding areas in tropical waters. An excess of males was recorded in the commercial whaling catches near breeding areas in the southern hemisphere, but no account of this was taken in developing population models, because it was believed that this bias was a result of whalers selecting against females with young calves. Here we demonstrate that the sex ratio of migrating humpback whales near a breeding area is highly skewed towards males. A biopsy study carried out in 1992 throughout the northward and southward migrations revealed a sex ratio of 2.4 males: 1 female in the population of humpback whales migrating along the east Australian coast (n = 180). A reanalysis of the catches made during commercial whaling in this and other areas of the southern hemisphere gave a sex ratio of the same order. The most plausible explanation, supported by some evidence, is that some females remain in the feeding areas throughout winter. The results reported here show that existing management models require major revision to take account of these findings.

The marsupial MHC: the tammar wallaby, Macropus eugenii, contains an expressed DNA-like gene on chromosome 1.

In the placental mammal major histocompatibility complex (MHC) three main families of class II genes, DR, DQ, and DP, have been recognized. Each family contains genes that code for one or more A- and B-chains. Recent evidence has indicated that a fourth family can be described, the DN/DO family. These four families arose sometime early in mammalian evolution. Our purpose was to deduce the MHC of an early mammalian ancestor of marsupials and eutherians. Using primers designed to conserved regions in exon 2 and exon 3 of the DQA gene we amplified an 830-bp band from the total genomic DNA of the marsupial, Macropus eugenii (tammar wallaby). However, sequence analysis of cloned genomic products showed that the primers had amplified three genes, two of which appeared to be alleles at one locus, while the other gene belonged to a closely related locus. Phylogenetic analysis showed that both these loci were most closely related to the human (HLA-DNA) and mouse (H-20a) DNA genes, with a bootstrap support of 78%. Expression of only one locus could be detected by RT-PCR from spleen RNA. In situ hybridization to tammar wallaby chromosomes mapped these genes to one region on the long arm of chromosome 1, indicating the position of the MHC in marsupials. Related A-chain genes were detected in monotremes, and human by southern blotting, and very faint bands were observed in the chicken. Hybridization with a tammar DNA-like gene on several marsupial species showed evidence of at least three DNA-like loci in the tammar wallaby, at least one in the koala, but none in the kowari. This indicates that the organization of the class II MHC may be more dynamic in marsupial than in placental mammals, but, in contrast to a previous study on the MHC of a marsupial, we cannot conclude that the class II gene families of placental and marsupial mammals evolved from different ancestral genes.

Rapid assessment of single-copy nuclear DNA variation in diverse species.

We investigated the use of PCR primers designed to conserved exons within nuclear DNA to amplify potentially variable regions such as introns or hypervariable exons from a wide range of species. We then explored various approaches to assay population-level variation in these PCR products. Primers designed to amplify regions within the histone H2AF, myoglobin, MHC DQA, and aldolase (ALD) genes gave clean amplifications in diverse mammals (DQA), and in birds, reptiles and mammals (aldolase, H2AF, myoglobin). The sequenced PCR products generally, but not always, confirmed that the correct locus had been amplified. Several primer sets produced smaller size fragments consistent with preferential amplification of intronless pseudogenes; this was confirmed by sequencing seal and reptile H2AF PCR products. Digestion with randomly selected four-base recognizing enzymes detected variation in some cases but not in others. In species/gene combinations with either low (e.g. seal H2AF, ALD-A) or high (e.g. skink ALD-1) nucleotide diversity it was more efficient to sequence a small number of distantly related individuals (e.g. one per geographic population) and from these data to identify informative or potentially informative restriction enzymes for 'targeted' digestion. We conclude that for studies of population-level variation, the optimal approach is to use a battery of primers for initial PCR of both mtDNA and scnDNA loci, select those that give clean amplifications, and sequence one sample from each population to (i) confirm gene identity, (ii) estimate the amount of variation and, (iii) search for diagnostic restriction sites. This will allow determination of the most efficient approach for a large-scale study.

Immunocytochemical and electrophoretic analyses of changes in myosin gene expression in cat posterior temporalis muscle during postnatal development.

Changes in myosin gene expression during the postnatal development of the homogeneously superfast kitten posterior temporalis muscle were examined using immunocytochemical techniques supplemented by pyrophosphate gel electrophoresis and gel electrophoresis-derived enzyme linked immunosorbent assay (GEDELISA) of myosin isoforms. The antibodies used were polyclonals directed against the heavy chains of superfast and foetal myosins and monoclonals against the heavy chains of slow and fast myosins. The fibres of the posterior temporalis in the newborn kitten stained almost uniformly with the anti-foetal myosin antibody and the largest of these fibres stained strongly for superfast myosin. A subpopulation of fibres staining for superfast myosin also stained lightly for slow myosin. These slow staining fibres were evenly distributed in the centres of muscle fibre bundles, reminiscent of primary fibres in limb fast muscle. During subsequent development, slow myosin staining disappeared and superfast myosin replaced foetal myosin so that by 50 days the muscle was virtually homogeneously superfast as in the adult. Fast myosin was never expressed at any stage. It is proposed that fibres staining transiently for slow myosin are superfast primary fibres which are homologous to fast primary fibres recently described in regions of limb muscles devoid of slow fibres in the matured animal. Other jaw-closing muscles have significant populations of slow fibres in the mature animal and it is postulated that there exists in these muscles a second class of jaw primary fibres, the slow primary fibres, in which slow myosin synthesis would be sustained in the adult. It is suggested that the myogenic cells of jaw-closing and limb muscles are of two distinct types preprogrammed to express different muscle genes.

Immunocytochemical and electrophoretic analyses of changes in myosin gene expression in cat limb fast and slow muscles during postnatal development.

Changes in myosin synthesis during the postnatal development of the fast extensor digitorum longus (EDL) and the slow soleus muscles of the kitten were examined using immunocytochemical techniques supplemented by pyrophosphate gel electrophoresis and gel electrophoresis-derived enzyme linked immunosorbent assay (GEDELISA) of myosin isoforms. The antibodies used were monoclonals against heavy chains of slow and fast myosins and a polyclonal against foetal/embryonic myosin. In both muscles in the newborn kitten, there was a population of more mature fibres which stained strongly for slow but weakly for foetal/embryonic myosin. These fibres were considered to be primary fibres. They formed 4.8% of EDL fibres and 26% of soleus fibres at birth, and continued to express slow myosin in adult muscles. The less mature secondary fibres stained strongly for foetal/embryonic myosin, and these could be divided into two subpopulations; fast secondaries in which foetal/embryonic myosin was replaced by fast myosin, and slow secondaries in which the myosin was replaced by slow myosin. At 50 days the EDL had a large population of fast secondaries (83% of total fibres) and a small population of slow secondaries which gradually transformed into fast fibres with maturity. The vast majority of secondary fibres in the soleus were slow secondaries, in which slow myosin synthesis persisted in adult life. There was a restricted zone of fast secondaries in the soleus, and these gradually transformed into slow fibres in adult life. It is proposed that the emergence of primary fibres and the two populations of secondary fibres is myogenically determined.

Interrelations of the rat's thalamic reticular and dorsal lateral geniculate nuclei.

Electrophysiological and neuroanatomical techniques have been used to study the properties of cells in the reticular nucleus of the thalamus (RNT) responsive to photic stimuli. In the rat these cells are located in a discrete region of the nucleus lying immediately rostral to the dorsal lateral geniculate nucleus (LGNd), where the visual field is represented in a retinotopic fashion. After injections of horseradish peroxidase (HRP) into this area, neurones labelled with reaction product were found in the LGNd and not in other thalamic relay nuclei. After HRP injections into the LGNd, labelled RNT cells were found only within the region which contains neurones responsive to photic stimuli. These observations suggest that there is a precise reciprocal relation between the two areas. Studies and comparisons of the responses of relay cells (P cells) in LGNd and cells in RNT to electrical shocks lead us to conclude that RNT cells receive their excitation mainly via those relay cells in LGNd which are themselves excited by fast-conducting retinal ganglion cell axons. Such cells in LGNd have phasic responses and concentric receptive fields while RNT cells have phasic responses and on/off fields and a comparison of the receptive field sizes of P cells and RNT cells suggests that only a small number of LGNd relay cells converge on to each RNT cells. Further, although a particular functional class of relay cells in LGNd (Y-type) is shown to provide the major input to visually responsive RNT cells, both Y type and W type relay cells are subject to their inhibitory control. These results furnish evidence that cells in the RNT have an important role in modulating the flow of visual information from the LGNd to cortex.

A correlation of receptive field properties with conduction velocity of cells in the rat's retino-geniculo-cortical pathway.

1. The receptive field properties and responses to electrical stimulation of 126 P-cells recorded from the dorsal lateral geniculate nucleus (LGNd) were studied in the hooded rat. 2. Eighty-five cells had a concentric (Kuffler, 1953) receptive field organisation (46 off-centre on-surround; 39 on-centre off-surround). Of the remaining cells 29 had co-extensive on/off excitatory discharge regions, nine had on-centres with suppressive surrounds and two cells gave on-responses but had no suppressive surround. One cell was identified as suppressed-by-contrast. 3. On the basis of the battery of tests developed for the identification of cell types in the cat's retina and LGNd, 35 of the cells with a Kuffler-type receptive field organisation were identified as Y-like. The majority of the remaining cells, both concentric and others, reminded us of the different subclasses of W-cells of the cat. Nine concentric cells in most of the tests exhibited X-like properties. 4. All of the Y-like cells were driven by relatively fast conducting retinal ganglion cell axons, comprising the t1 conduction velocity group. The majority of the remaining cells were driven by slower axons comprising t2 or t3 conduction velocity groups. 5. Thus, in the rat, as in other mammalian species studied so far, there is a correlation between the conduction velocity groups in the retino-geniculo-cortical pathway and the functional groups based on the cells' receptive field properties. There seem to be functional equivalents of the cat's Y- and W-cell classes but evidence for a distinct X-like class of cells is lacking.