EGG ACTIVATION AND PARTHENOGENETIC REPRODUCTION IN INSECTS

1. Many insects reproduce by parthenogenesis. In one of the largest orders of the animal kingdom, the Hymenoptera, most of its members reproduce by arrhenotokous parthenogenesis. Egg activation in parthenogenetic animals obviously cannot be caused by fertilization of the egg. The question of what initiates egg development in parthenogenetically reproducing animals has been studied for a few insect species and is discussed in this article. 2. The grasshopper Melanoplus differentialis is one of several Orthoptera displaying accidental parthenogenesis. In this species, egg laying provides the stimulus to the completion of meiosis and start of embryonic development in unfertilized and probably also in fertilized eggs. The same holds true for the dipteran insect Drosophila melanogaster which exhibits rudimentary parthenogenesis, and for D. mercatorum showing accidental parthenogenesis. The precise way in which oviposition affects the egg is unknown. 3. The stick insect Carausius morosus reproduces by obligatory thelytoky. The triggering factor for removal of the meiotic block and initiation of embryonic development is oxygen from the air which penetrates to the egg through the micropyle immediately after oviposition. The oviposition act itself is not necessary for activation of the egg. 4. Comparative studies of the different types of oogenesis in the dipteran insect Heteropeza pygmaea show that in paedogenetically developing follicles meiotic arrest in prophase is of very short duration and a meiotic block at the end of oogenesis is absent. It is suggested that in this case triggering events for egg development are dispensable. On the other hand, under certain experimental conditions a meiotic block can be established in some of these follicles. 5. Investigations on the Ichneumonid wasp Pimpla turionellae have shown that unfertilized, male-determined eggs - and most likely also fertilized, femaledetermined eggs - are activated by mechanical stress exerted on the eggs during natural or imitated oviposition. This mechanical stress, in addition, activates a streaming system which is independent of meiotic completion and nuclear multiplication. Egg activation by egg distortion is also found in the Pteromalid species Nasonia vitripennis and occurs presumably in many other Hymenoptera. 6. Carausius morosus, Pimpla turionellae and Nasonia vitripennis are species with parthenogenetic reproduction for which the natural factors responsible for the initiation of egg development have been identified. The cases of Pimpla turionellae and Nasonia vitripennis are of particular interest because of the feasibility of artificially imitating the natural activating mechanism. 7. It is concluded that apart from fertilization various events at oviposition may trigger egg development. In addition, the occurrence of rudimentary parthenogenesis in many sexually reproducing animal species suggests that sperm entry and fertilization may frequently be necessary for the continuation of egg development rather than for its initiation.     

A BREEDING BIRD CENSUS AND NESTING SUCCESS IN CENTRAL AMERICA

In a park-like area of 3.75 acres adjoining primary forest in Costa Rica, at an altitude of 2,500 ft., 83 nests, made by about 49 pairs of birds and two single females, were found in one year. Over a period of 20 years, 64 species were recorded as nesting in this same area. The difficulties of learning the actual rate of success of nesting birds are discussed, and it is concluded that, in view of the impossibility of assessing the effects of visits of inspection to nests in natural habitats, statements of breeding success are at best rough approximations of what happens in the absence of an observer. In the area of the census, nest-success (the proportion of nests in which at least one egg was laid that produced at least one living fledgling) was 38–53% in four different years. During the four years 41 % of 208 nests were successful. Of 756 nests of 23 species of altricial birds of the Central American lowlands that build open or roofed nests in clearings and second-growth, 37% were successful. When the computation is restricted to nests found before the last egg was laid (class B nests), 35% of 434 nests were successful, and 30% of 883 eggs produced living fledglings. In the neighbouring forests, nesting success was much lower, only 23.5% of 136 open or roofed nests producing at least one fledgling. Many forest birds increase their chances of success by entering neighbouring clearings to breed, but few open-country birds build their nests in the forest. In both forest and clearings, hole-nesting birds in Central America are much more successful than open-nesters, as has been found also in the North Temperate Zone. A comparison of the results of a single season's observations in each of six Central American localities shows an increase of nesting success with altitude. In lowland Panama, the nest-success was only 21%, in the Subtropical Zone of Costa Rica 53%, and in the altitudinal Temperate Zone of Guatemala 55%. The effect of altitude is complicated by differences in the amount of forest in the localities chosen for study, as well as by other factors difficult to assess. In both the tropics and the North Temperate Zone, nest losses are substantially higher in woodland than in man-made habitats, evidently because there are fewer predators in the latter; but, even in clearings in Central America, nesting success was considerably lower than it was found to be in numerous studies in the North Temperate Zone. The difference may, however, reflect the greater “wildness” of the localities where the writer's studies were made, rather than a true contrast between tropical and temperate zone conditions. Snakes appear to be of the greatest single cause of nest losses in tropical America, but mammals, a few predatory birds, ants, and possibly even bats, destroy many eggs and young. Since small broods and heavy predation permit only a small annual contribution to the adult population, it is evident that, in order to maintain the species, adults must enjoy fairly long lives. Recent statistical studies support this theoretical conclusion.     

SEXUAL REPRODUCTION AND INTERCROSSING IN VOLVULINA STEINW1

Gametes of Volvulina steinii bear near-apical mating papillae. Zygospore germination yields a single biflagellate cell that develops into a colony xuhose asexual progeny are all of the same mating type. Backcrossing of clones of progeny indicated a 1 :1 ratio of mating types among the progeny. Of 20 clones from a number of localities, none was homothallic and 3 showed no matins: reaction. Mating reactions of clones crossed in all possible combinations indicated the presence of 2 sexually isolated groups of clones producing smooth-walled zygospores and 1 group that produced spiny-walled zygospores. In the latter group weak and nonreciprocal mating reactions occurred in some clone combinations. Failure of germination of spiny-walled zygospores from certain crosses suggests further subdivision into genetically isolated groups.     

SYNCHRONOUS FLUCTUATIONS IN POPULATIONS OF SOME RAPTORS AND THEIR PREY

Raptorial birds which depend on a small number of prey species, especially those living in open habitats in the tundra, semi-desert and desert belts of the Palaearctic, undergo local fluctuations in numbers which are synchronous with the fluctuations in numbers of their main prey species. In this they contrast with the asynchronous or lagging type of predator-prey oscillation which has received wider attention in the literature.
Evidence from ringing recoveries shows that the species which fluctuate locally in synchrony with their prey tend to move greater distances from their birth-place or previous breeding place than those with more stable populations. The former species also tend to be split into fewer sub-specifically distinct forms than the latter. It is argued that the species which are subject to a fluctuating food supply have evolved, as an important adaptation, the ability to undertake more or less extensive 'searching migrations', which enable them to find and settle in areas of adequate food supply, often far removed from the previous breeding area. In the longer evolutionary aspect, this ability may be a factor promoting food specialization.
The synchronous population fluctuations of raptorial birds with their prey are compared with the asynchronous or lagging oscillations of carnivorous mammals.     

现代农业中的激光技术和激光育种

现代农业中的激光技术是一项新技术。激光农用的开拓领域相当广泛。激光育种在我国始于70年代初,据不完全统计到1986年为止,已育成粮、油、棉、蔬菜、果树、桑、蚕等新品种(系)36个,对农业生产起到了良好作用。为了在现代农业中发挥激光技术的应用优势,从育种的角度来说,应该在加强激光生物学多学科之间合作的前提下,大力开展激光生物工程的研究,这也是激光应用于农业发展的希望所在。     

SEXUAL REPRODUCTION IN CLOSTERIUM MONILIFERUM AND CLOSTERIUM EHRENBERGII1

Sexual reproduction in C. moniliferum is described for the first time. The morphology of conjugation is quite like that of C. ehrenbergii. Homothallic strains of both species usually produce single zygospores between daughter cells that have just divided. However, 2 homothallic clones of C. moniliferum form twin zygospores between conjugants which have paired before division and conjugation. This has not been observed in C. ehrenbergii. Heterothallic strains of both species form twin zygospores in the same manner. Heterotfiallisrn seems a well-established feature in both species. Germination and the survival of 2 products of meiosis arc typical of other desmids which have been investigated.     

NUTRITION AND SEXUAL REPRODUCTION IN THE WATER MOLD ACHLYA

The entire sexual reproductive progression in the heterothallic water mold Achlya ambisexualis Raper occurs in the absence of exogenous nutrients. Mated male and female mycelia, previously grown on a chemically defined medium, can utilize endogenous materials for the formation and maturation of sexual reproductive structures. Under these conditions the initiating sexual hormone antheridiol, produced by the female, exerts its morphogenetic effect on male mycelium in the absence of exogenous nutrients.     

STOCHASTICITY IN A PREDATOR-PREY SYSTEM UNDER PERIODIC ENVIRONMENTAL FLUCTUATIONS

In the Present PaPer,using as examPles two models〔I,2〕of thedynamies of the quantity of a Predator一Prey system sueh as the generalizedLotka一Volterra'mode一under Periodie environmental fluetuations,we showthe existenee of eomPlex stoehastie regimes of behaviour.our investigationha 5 been earried out qualitatively〔3〕(ItemsZ。一40)based on meehan-isms of loeal eodimenson two bifureations〔4,5〕and on Perturbationtheory〔6一