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LUYKX, P. 1990. A cytogenetic survey of 25 species of lower termites from Australia. Genome, 33: 80-88.A survey of 25 species of lower termites (families Mastotermitidae, Termopsidae, and Kalotermitidae) in Australia revealed that centric fusions are a common theme in karyotype evolution in these insects. All but one of the species studied have a basic XX/XY mechanism of sex determination, secondarily complicated in about a third of a species by centric fusions between autosomes and sex chromosomes. There is no obvious relationship between systematic position and presence or absence of these fusions. Fusions between Y chromosomes and autosomes were more common than fusions between X chromosomes and autosomes, in accord with the prediction of the hypothesis that differential selection between the two sexes is the basis for the spread of sex-linked fusions. The absence of these fusions in many species does not favor the idea that a high degree of sex linkage is a necessary condition for the establishment or maintenance of eusocial behavior in termites. The difference in the mechanism of sex determination from that of cockroaches (XX/XO) argues against the evolutionary derivation of termites from ancestral cockroaches; derivation of both groups from some common ancestor with XX/XY sex determination is more likely. LUYKX, P. 1990. A cytogenetic survey of 25 species of lower termites from Australia. Genome, 33 : 80-88.Une etude cytogenetique de 25 especes de termites des familles de classification inferieure, les Mastotermitidae, les Termopsidae et les Kalotermitidae, a revele que les fusions centriques constituent un theme commun relatif a l'evolution du caryotype chez ces insectes. Toutes les espkces etudiees, sauf une, possedent un mecanisme de base de determination des sexes XX/XY, avec complications secondaires chez environ un tiers des espkces en raison de fusions centriques entre des autosomes et les chromosomes du sexe. I1 n'y a pas de relation evidente entre la position systematique et la presence ou l'absence de ces fusions. Les fusions entre les chromosomes Y et les autosomes ont ete plus generales que les fusions entre les chromosomes X et les autosomes, en accord avec la prediction de l'hypothese que la selection differentielle entre les deux sexes est la base de la distribution des fusions liees au sexe. L'absence de ces fusions chez plusieurs des especes ne favorise pas l'idee qu'un degre eleve de linkage des sexes soit une condition necessaire pour l'etablissement ou le maintien d'un veritable comportement social chez les termites. La difference du mecanisme de determination des sexes par rapport a celui des blattes (XX/XO) constitue un argument a l'encontre d'une derivation des termites a partir de blattes ancestrales; la derivation des deux groupes a partir d'un ancEtre commun avec determination des sexes XX/XY est plus plausible.

Summary.The contractile vacuole (CV) cycle of Chlamydomonas reinhardtii has been investigated by videomicroscopy and electron microscopy. Correlation of the two kinds of observation indicates that the total cycle (15 s under the hypo-osmotic conditions used for videomicroscopy) can be divided into early, middle, and late stages, In the early stage (early diastole, about 3 s long) numerous small vesicles about 70-120 nm in diameter are present. In the middle stage (mid-diastole, about 6 s long), the vesicles appear to fuse with one another to form the contractile vacuole proper. In the late stage (late diastole, also about 6 s long), the CV increases in diameter by the continued fusion of small vesicles with the vacuole, and makes contact with the plasma membrane. The CV then rapidly decreases in size (systole, about 0.2 s). In isosmotic media, CVs do not appear to be functioning; under these conditions, the CV regions contain numerous small vesicles typical of the earliest stage of diastole. Fine structure observations have provided no evidence for a two-component CV system such as has been observed in some other cell types. Electron microscopy of cryofixed and freeze-substituted cells suggests that the irregularity of the profiles of larger vesicles and vacuoles and some other morphological details seen in conventionally fixed cells may be shrinkage artefacts. This study thus defines some of the membrane events in the normal contractile vacuole cycle of Chlamydornonas, and provides a morphological and temporal basis for the study of membrane fusion and fluid transport across membranes in a cell favorable for genetic analysis.

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