摘要:

The evolution of genetic systems from the molecular level up to that of reproductive mechanisms is discussed and previous views are criticized. The variables of the meiotic process are examined with special regard to cytoplasmic phenomena.

ISSN:0373-4137    

DOI:10.1080/11250008409439455

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

In the marine invertebrates which practise external fertilization, the spermatozoon is a flagellate cell, possessing an acrosomal complex made up of acrosome and perforatorium. In the animal phyla which adapted to the land after having achieved the internal fertilization, the spermatozoon acquires an elongate shape and develops accessory cytoskeletal organelles in the tail. In the highest forms, the flagellum seems to have become too long for a perfect motility; in such a condition, aberrant axonemal patterns can appear and aflagellate spermatozoa can also be found. At the apex of the terrestrial phyla, alternative motile apparatuses are achieved and also the acrosomal complex varies, according to the different structures of egg envelopes.

ISSN:0373-4137    

DOI:10.1080/11250008409439456

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

In this paper, the evolution of egg envelope is examined from a structural point of view. The structurally simplest eggs are found in Sponges and Cnidarians where reproduction is ensured by external fertilization. In these instances, the oocyte plasma membrane is protected by only a thin glycocalyx. A more structurally complex envelope is present in triblastic Metazoa where the vitelline membrane makes its first appearance.

ISSN:0373-4137    

DOI:10.1080/11250008409439457

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

In this paper we discuss the events leading to the recognition and fusion of sperms and eggs. In the course of evolution a glycoprotein coat, the vitelline coat, has differentiated around the egg. This coat carries the sperm receptors and is a product of the oocyte. The receptors are glycoprotein molecules; in most of the species studied fucose appears to be the most important component as regards receptor function. In most, though not in all, animals the interaction of the spermatozoa with the sperm receptors triggers the acrosome reaction. Some observations further suggest that the vitelline coat may also serve to repress the metabolism of the unfertilized egg.

ISSN:0373-4137    

DOI:10.1080/11250008409439458

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

Vertebrate reproductive modes, which are characterized by egg and development type, have been compared and, in some cases, re-defined. Reproductive strategies regulate reproductive effort. Reproductive modes and strategies have been selected in species history: more anciently and with more conservative characters the former; more recently and with some flexibility in response to environmental changes the latter.

ISSN:0373-4137    

DOI:10.1080/11250008409439459

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

The reproductive activity in most Metazoa is cyclic. Such cyclicity is synchronized by different types of environmental factors, in particular by temperature (heterotherms) and photoperiod (homeotherms).

ISSN:0373-4137    

DOI:10.1080/11250008409439460

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

In species with sexual reproduction, one sex, more often female, initially contributes greater parental investment, since she produces gametes that are fewer in number but bigger in size. This leads to intrasexual competition among individuals of the opposite sex to achieve reproduction. The rise of polygamic reproductive systems is a direct consequence of sexual selection, as Darwin has already stated. During evolution, polygamy appeared before monogamy. The latter probably evolved as an adaptation to harsh habitats, where survival was possible only for two individuals who cooperated during the reproductive period.

ISSN:0373-4137    

DOI:10.1080/11250008409439461

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

Sessile animals show peculiar reproductive strategies which are linked to their speciation mechanisms. The A. considers the adaptive role of both asexual and sexual reproduction. Asexual reproduction through budding builds colonies — coloniality being an important adaptation for substrate occupation. In the solitary sessile animals coloniality may be replaced by gregariousness. Sexual reproduction, often accompanied by hermaphroditism and sex reversal, has a basic function, besides the maintenance of an intraspecific genetic variability, in species dispersal and habitat selection through the recruiting larvae. The high degree of morphological variability, which is an adaptation to sessile condition, is reached — as in plants — not only through genetic polymorphism depending on sexual reproduction but also through phenotypic plasticity. Sessile macrofauna probably developed originally as an adaptive condition to unstable and scattered substrata, under the influence of unpredictable physico-chemical factors and followed a generalist (r-selected), opportunistic strategy. Later on, however, also in relation to the evolution of coloniality, a more selective, specialist (K-selected) strategy has been developed. This hypothesis finds support in some paleontological data. To-day the two opposite, r-selectel and K-selected, strategies are both present. In conclusion, the reasons for the evolutionary success of marine sessile animals, which still constitute the bulk of some of the more productive world ecosystems as the coral reefs, may be found not only in their extraordinary trophic plasticity but also in the large array of their sexual and asexual reproductive strategies.

ISSN:0373-4137    

DOI:10.1080/11250008409439462

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

Copulation is considered to occur when both of two conditions are met: (a) spermatozoa are deposited by a specialized male organ in the female genital tract or in female structures leading to the site of fertilization, and (b) fertilization is internal. As such, copulation is known to occur in eight of the twenty-one animal phyla, including both the simplest and most complex organisms; it can be inferred to have developel inlependently several times during the course of evolution; it is notably associated with a terrestrial habitat or a parasitic status. Many morphological details of copulatory organs that differ between species have no obvious functional significance and probably reflect variation under low selection pressure.

ISSN:0373-4137    

DOI:10.1080/11250008409439463

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

ISSN:0373-4137    

DOI:10.1080/11250008409439464

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor