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Biomedical subjects

B John

Publications and source records attributed to B John.

At least 55 records · Page 3Linked to original sources
PubMed1977

Cytogenetic components of reproductive isolation in Trimerotropis thalassica and T. occidentalis.

The grasshopper Trimerotropis thalassica (Bruner) has a diploid count of 2n=23 male (XO), 24 female (XX). The two largest autosomes pairs are regularly metacentric, a consequence of fixed pericentric inversions. The X-chromosome is also a fixed metacentric. The remaining nine pairs of autosmes are polymorphic for floating percentric inversions so that the complement consists of a mixture of telocentric and metacentric members. Trimerotropis occidentalis (Bruner) is polymorphic for comparable inversions in only two of its autosome pairs and has a telocentric X. It is however, unique among the species of the genus Trimerotropis in having only 21 chromosomes in its male diploid set in all the populations so far studied. A single male found in a mixed population of these two species at Jasper Ridge, Stanford University, was characterized by the count 2n=22 male. In both this respect of and in its phenotype it was intermediate in character, representing a natural F1 hybrid between the two species. Cytogenetic analysis of this hybrid male indicated that occidentalis is differentiated from thalassica only is respect of a single tandem translocation. This has involved two of the telocentric elements of thalassica which have fused into a single composite telocentric partly homologous with each of the smaller progenitors. Although potentially capable of forming a multiple of three, one or other of the progenitor chromosomes regularly fails to pair with the tandem product in the hybrid so that one or more univalents invariably occur. These, by lagging, prevent cytokinesis and subsequently lead to the formation of macrospermatids which inevitably produce a measure of sterility. It is argued that this sterility provides a basis of reproductive isolation.

Animals↗
PubMed1976

Myths and mechanisms of meiosis.

A comparative analysis of the meiotic secquence in a wide variety of organisms indicates there is no convincing evidence that: (1) Premeiotic pairing plays any role in the synapsis of homologues. (2) Heterochromatic association facilitates homologous pairing. (3) Chiasmata ever form within segments which are positively heteropycnotic at zygotenepachytene. (4) Localisation of chiasmata depends on prior localisation of pairing or on the occurrence of euchromatin-heterochromatin boundaries. (5) Prior association of centromeres plays any role in determing co-orientation. (6) Any form of supra-chromosomal organisation exists involving permanent association between the members of a haploid complement, and (7) Unequal progeny ratios recovered from structurally modified Drosophila complements arise as a consequence of distributive pairing.--On the other hand there is good evidence that: (1) Interlocking of bivalents can occur regularly in species with a chiasma frequency sufficiently high to regularly produce ring bivalents and in which the chiasmata are localised to the ends of the bivalent. (2) Some forms of terminal association cannot represent terminalised chiasmata. (3) U-type exchanges present at diplotene result from errors in crossing over. (4) Pairing and chiasma formation are not necessary for coorientation, and (5) at least some types of elastic constrictions present at first metaphase represent extended nucleolar organisers.

Animals↗
PubMed1976

Case report: lithium in reactive depression.

A 25-year old woman suffering from severe reactive depression was treated with lithium carbonate 1500 mg per day. On this regime she showed a very good improvement, and after 4 weeks of in-patient treatment was free of symptoms. At 6-months' follow-up there was no recurrence of symptoms; her serum lithium levels were within therapeutic range (0.6 to 1.2 mEq/1).

Adjustment Disorders↗
PubMed1975

The cytogenetic structure of Tasmanian populations of Phaulacridium vittatum.

In 8 out of 20 Tasmanian populations of Phaulacridium vittatum from 0.3-11.0 percent of the males carried a single supernumerary chromosome. In such males the X and B univalents are both heteropycnotic at first prophase of male meiosis and associate with another in a non-homolgous manner in about two-thirds of the diplotene cells examined. In all 56 B-containing individuals studied, however, these associations lapse by first metaphase and the X and the B move at random with respect to one another at first anaphase. The supernumerary in this species is stable and only 5 of the 56 individuals with supernumeraries showed evidence of B-chromosome non-disjunction in the pre-meiotic mitoses. Since there was no other evidence for the loss or gain of supernumeraries in the male line it is clear that B-transmission is regular in the males of this species. There were significant differences between population with respect to mean cell chiasma frequency but there was no significant effect of the B-chromosome on this metric. Additionally a comparison of mean cell chiasma frequency in follicles with and without supernumeraries from a mosaic individual shows no significant difference.

Animals↗
PubMed1975

Causes and consequences of Robertsonian exchange.

At least two types of Robertsonian exchange are now known in the acrocentric chromosomes of man. Both types involve breakage in the arms adjacent to the centromere. Evidence is presented for a third type of exchange, one involving breakage within the centromere itself, in the grasshopper Percassa rugifrons. In this species, which is regularly homozygous for a single fusion metacentric, the eighteen rod autosomes have small but pronounced granules at the centric end of the chromosome. When C-banded these granules show differential Giemsa staining and appear to represent centromeric chromomeres; these chromomeres are lacking in the metacentric fusion product. Equivalent fusions may have occurred in some mammal species too and possible examples of this are discussed in sheep and mice. The Percassa fusion has led to a modification in both the frequency and the distribution of chiasmata as judged by a comparison of these properties in the metacentric relative to the two next smallest rod equivalents. Comparable modifications are known to occur in other naturally occurring fusions but these changes are certainly not automatic consequences of fusion since they are not shown in at least some newly produced fusion mutants.

Animals↗
PubMed1975

Evidence that the deferential vein acts as a local transport system for androgen in the rat and the dog.

Ligation of the deferential vein of the rat has been shown to bring about a reduction in the androgen-dependent activity of the prostatic RNA polymerase enzyme. In the dog, the content of androgens in this vein is of the order of that found in the testicular vein and many times higher than that of peripheral plasma. It seems that the cauda epididymidis alone is sufficient to maintain the ipsilateral lobes of the prostate and seminal vesicles providing an intact ductus deferens and contralateral testis are retained. The results are discussed in relation to the hypothesis that the deferential vein of the dog and rat may serve as a direct transporting system for androgens from the epididymis to the prostatic complex.

Androstanes↗