Reproduction, asexual sexual

We should note once again the general mechanism by which resistance appears in target species is natural selection. Many factors may accelerate or slow this process a growth in frequency or number of resistance alleles their dominance, ability to penetrate, and expressivity their interaction with various genes the speed with which generations are replaced the number of individuals in each generation the character of the reproductive system (sexual or asexual) and a host of other factors [113]. 

Much of the behaviour of humans and society as a whole is designed to provide an opportunity for a sperm and an ovum to meet in the uterus. However, sexual reproduction is not the only means of reproduction asexual reproduction occurs frequently in the animal kingdom and in many tissues in order to increase the number of cells during growth or to maintain the number due to cell death. The question arises, therefore, why sexual reproduction has arisen at all since it is very expensive in the use of resources for example, in the process of selecting a suitable mate, and in the requirement for males, whose only function in some species is fertilisation of the females, yet they use resources that could be used by the females and by the offspring. Perhaps because of this, a few species of vertebrates (e.g. some lizards) have retained the option for asexual reproduction. This process is known as parthenogenesis. Mammals, however, have lost this option. This... 

It should be noted that the closely related species, M. lineatus, does not undergo asexual reproduction in either the tetrathyridium or adult stage it too, has been successfully cultured to maturity in vitro (382). The possible factors inducing asexual/sexual differentiation in both these species, and in other cestodes, is further considered in Chapter 10, where in vitro systems are reviewed. 

The Jerusalem artichoke can reproduce by two primary means. It can reproduce and colonize an area by the allocation of photosynthate and nutrients into both asexual (tubers and, to a lesser extent, rhizomes) and sexual (seed) reproductive organs. Flexibility in the amount of resources allocated between sexual and asexual means of reproduction confers a selective advantage in that conditions that inhibit or block sexual production (lack of pollen, herbivory of floral structures, undesirable weather) allow increased allocation to asexual reproduction. Artificially reduced allocation of resources to sexual reproduction, for example, results in a substantial increase in those allocated to asexual means. With flower bud removal, more (82 vs. 69) and larger (4.4 vs. 3.8 g) tubers were formed per plant than those with unlimited sexual reproduction (Wesdey, 1993). Total biomass was not altered, potentially indicating a relatively complete diversion of resources to asexual reproduction when sexual reproduction is blocked. From a reproductive standpoint, the risk of making it to the next season is high with sexual reproduction and relatively low with asexual reproduction. Increased investment in tubers increases the opportunity for sexual reproduction in the future. 

There are several classes of reproduction, and we start with asexual vs. sexual reproduction. Asexual reproduction is the creation of new individuals whose genes all come from one parent without the fusion of egg and sperm (Campbell et al., 1999). That means that each offspring of the process is a genetic clone of the parent. 

Throughout the animal kingdom from protozoa to human beings sexual reproduction predominates. It is true that there are about 1000 species that reproduce asexually.242 243 Among them are 350 species of allfemale rotifers242 and even a species of tiny mites, all of which are haploid females.244 Nevertheless, sex seems to have conferred some advantage on most species. There are two thories that may explain this ... 

The vegetative body is a thallus. It consists of filaments about 5 pm in diameter which are multi-branched or spread over or into the nutrient medium. The filaments or hyphae, can be present without cross walls as in lower fungi or divided into cells by septa in higher fungi. The total hyphal mass of the fungal thallus is called the mycelium. In certain situations during transition between asexual and sexual reproduction, various other tissue structures are formed, e.g. plectrenchyma (mushroom flesh). 

Ziegler, J. R. and Kingsbury, J. M., Cultural studies on the marine green alga Halicystis parvula-Der-besia tenuissima. I. Normal and abnormal sexual and asexual reproduction, Phycologia, 4, 105, 1964. 

This species has a life cycle similar to that of M. corti but, unlike it, does not undergo asexual reproduction in either the larval or adult stage. The adult of this species has also been grown in vitro from the tetrathyridium, but, in this case, the induction of sexual development appears to be related to the presence and activity of trypsin, for differentiation was inhibited or decreased by the presence of soybean trypsin inhibitor (382). 

The diameter of the head of the inflorescence ranges from 1.3 to 1.8 cm (six clones) and the mean number of seed per plant from 0.45 to 163 (Swanton, 1986). In wild clones, 9% of the biomass by the end of the season had been allocated to the flowers and fruit (Westley, 1993). If sexual reproduction was blocked, there was a substantial increase in biomass allocated to asexual means of reproduction in the form of more and larger tubers. Flowers in the late fall (November 1, Ontario, Canada) contained 2.46% N, 0.51% P, 2.02% K, 1.21% Ca2+, and 0.68% Mg2+ (Swanton and Cavers, 1989). 

Further, since the reproduction test with Daphnia only comprises asexual reproduction, this also means that investigations focusing on sexual reproduction will be lacking for substances produced up to 1,0001 per producer and year. For substances produced above this production volume, the reproduction tests using earthworms and chironomids may however be used to generate adequate population data (see Table 6.1 for OECD test guidelines concerned with reproduction available for risk... 

What value will sexual reproduction have in the face of the certainties of asexual reproduction ... 

Unicellular More than a single chromosome per cell Heterotrophic or photoautotrophic nutrition Asexual or sexual reproduction protists Protoctist... 

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