Somatic and germ-cell sex in mammals

The phenotypic sex of an individual mammal is determined by the sex of its gonads, i.e. testes or ovaries. This in turn is determined by the presence or absence of a small region of the Y chromosome, located near the X-Y pairing region in man and on the short arm of the Y chromosome in the mouse. The testis-determining region of the Y appears to exert its primary effect by directing the supporting-cell lineage of the gonad to differentiate as Sertoli cells, acting at least in part cell-autonomously. The phenotypic sex of a germ cell, i.e. whether it undergoes spermatogenesis or oogenesis, is determined at least in the mouse by whether or not it enters meiotic prophase before birth. This depends not on its own sex chromosome constitution, but on its cellular environment. A germ cell in or near normal testis cords (made up mainly of Sertoli cells) is inhibited from entering meiosis until after birth; one that escapes this inhibition will develop into an oocyte even if it is in a male animal and is itself XY in chromosome constitution.     

Books

Book reviewed in this article:
Digital Futures: Living in a Dot-Com World, edited by James Wilsdon.     

Transitivity,pattern-reversal,engulfment and duality in exchange-type cell aggregation kinetics

Simulations of cell-sorting phenomena in embryogenesis describable by means of the differential adhesion mechanism are further studied using the exchange model. In this study, more than two cell-types are employed and results nicely complement the successes of earlier work with only two cell-types. Again, it is noticed that “central clumping” does not usually appear in final distributions. In addition, the phenomena of transitivity, pattern-reversal and the engulfment of one tissue by another are studied. The first is shown to be a trivial consequence of Steinberg's stochastic version of differential adhesion. Also, it is shown that the exchange model successfully simulates pattern-reversal by the reversal of the relative adhesion values due to the time-in-culture reversal of the effect of adhesivity of trypsin-assisted dissociation of tissues. The connection between the occurrence of pattern reversal and the absolute and relative positions of tissue types in the adhesion hierarchy is explained in terms on the exchange model. It is found that the model, as it stands, does not seem to give a satisfactory basis for the simulation of engulfment. Suggestions for the extension of the exchange principle are made which will perhaps rectify this limitation. It is felt that any real system which exhibits engulfment will also exhibit central clumping, and conversely. The importance of the figure-ground ambiguity we have called “duality” is again stressed. If this ambiguity is not taken into account in experimental work, interpretation of many results on cell-sorting are rendered meaningless.     

Observations on the life cycle and establishment ofCochylis atricapitana (Lep: Cochylidae), a moth used for biological control ofSenecio jacobaea in Australia

Larvae of the mothCochylis atricapitana (Stephens) are monophagous leaf, crown, stem or bud borers of ragwort,Senecio jacobaea L. (Asteraceae). In the present investigation, aspects of the life cycle ofC. atricapitana were determined. Moths ofC. atricapitana lay an average of 158 eggs/female with as many as 355 eggs being laid by a single female. The majority of eggs are laid individually along the primary and secondary veins on the underside of ragwort leaves. Egg incubation ranges from 4.2 days at 30°C to 14.4 days at 15°C. At a constant 23°C under a 16 hour photoperiod,C. atricapitana takes approximately 40 days to complete a generation. Caterpillars make their way to young, actively growing ragwort shoots or buds, and begin mining into the plant tissue, boring into the leaf, crown, stem or bud.C. atricapitana has five larval instars and enters diapause as a final instar larva. In southern Victoria, moths ofC. atricapitana fly from late September through to the beginning of February. Adults emerge after overwintering towards the end of spring or beginning of summer.C. atricapitana has established at two sites while larvae, or signs of damage have been observed at approximately 52% of release sites.