By What Mechanism Does A Diploid Animal Grow After Fertilization?
Chapter vii: Introduction to the Cellular Basis of Inheritance
7.i Sexual Reproduction
Learning Objectives
By the end of this department, you volition be able to:
- Explain that variation amidst offspring is a potential evolutionary reward resulting from sexual reproduction
- Describe the three different life-bike strategies among sexual multicellular organisms and their commonalities
- Understand why you lot could never create a gamete that would be identical to either of the gametes that made yo
Sexual reproduction was an early evolutionary innovation after the appearance of eukaryotic cells. The fact that most eukaryotes reproduce sexually is prove of its evolutionary success. In many animals, information technology is the merely mode of reproduction. And yet, scientists recognize some real disadvantages to sexual reproduction. On the surface, offspring that are genetically identical to the parent may appear to be more advantageous. If the parent organism is successfully occupying a habitat, offspring with the same traits would exist similarly successful. There is too the obvious benefit to an organism that can produce offspring by asexual budding, fragmentation, or asexual eggs. These methods of reproduction practise not require another organism of the opposite sex activity. At that place is no need to expend energy finding or attracting a mate. That energy tin be spent on producing more offspring. Indeed, some organisms that lead a solitary lifestyle have retained the ability to reproduce asexually. In addition, asexual populations merely have female person individuals, and so every individual is capable of reproduction. In dissimilarity, the males in sexual populations (one-half the population) are not producing offspring themselves. Because of this, an asexual population can abound twice every bit fast as a sexual population in theory. This means that in competition, the asexual population would accept the reward. All of these advantages to asexual reproduction, which are besides disadvantages to sexual reproduction, should mean that the number of species with asexual reproduction should be more common.
However, multicellular organisms that exclusively depend on asexual reproduction are exceedingly rare. Why is sexual reproduction so mutual? This is ane of the of import questions in biology and has been the focus of much inquiry from the latter half of the twentieth century until now. A likely explanation is that the variation that sexual reproduction creates among offspring is very important to the survival and reproduction of those offspring. The only source of variation in asexual organisms is mutation. This is the ultimate source of variation in sexual organisms. In addition, those different mutations are continually reshuffled from ane generation to the adjacent when unlike parents combine their unique genomes, and the genes are mixed into dissimilar combinations by the process of meiosis. Meiosis is the partition of the contents of the nucleus that divides the chromosomes among gametes. Variation is introduced during meiosis, every bit well as when the gametes combine in fertilization.
The Red Queen Hypothesis
There is no question that sexual reproduction provides evolutionary advantages to organisms that employ this mechanism to produce offspring. The problematic question is why, even in the face of fairly stable conditions, sexual reproduction persists when it is more difficult and produces fewer offspring for individual organisms? Variation is the effect of sexual reproduction, but why are ongoing variations necessary? Enter the Red Queen hypothesis, commencement proposed by Leigh Van Valen in 1973. 1 The concept was named in reference to the Red Queen's race in Lewis Carroll'south book, Through the Looking-Glass, in which the Red Queen says 1 must run at full speed but to stay where ane is.
All species coevolve with other organisms. For example, predators coevolve with their prey, and parasites coevolve with their hosts. A remarkable example of coevolution between predators and their prey is the unique coadaptation of night flight bats and their moth prey. Bats notice their prey by emitting high-pitched clicks, but moths have evolved unproblematic ears to hear these clicks so they can avert the bats. The moths accept likewise adapted behaviors, such every bit flying away from the bat when they first hear it, or dropping all of a sudden to the ground when the bat is upon them. Bats have evolved "quiet" clicks in an attempt to evade the moth'due south hearing. Some moths have evolved the ability to respond to the bats' clicks with their own clicks every bit a strategy to misfile the bats echolocation abilities.
Each tiny advantage gained past favorable variation gives a species an edge over close competitors, predators, parasites, or even prey. The only method that will allow a coevolving species to proceed its ain share of the resource is besides to continually better its power to survive and produce offspring. As i species gains an reward, other species must also develop an advantage or they will be outcompeted. No single species progresses too far alee considering genetic variation among progeny of sexual reproduction provides all species with a mechanism to produce adapted individuals. Species whose individuals cannot keep up become extinct. The Red Queen's catchphrase was, "It takes all the running yous can practise to stay in the same identify." This is an apt description of coevolution between competing species.
Life Cycles of Sexually Reproducing Organisms
Fertilization and meiosis alternate in sexual life cycles. What happens between these 2 events depends on the organism. The procedure of meiosis reduces the resulting gamete's chromosome number by one-half. Fertilization, the joining of two haploid gametes, restores the diploid condition. There are three main categories of life cycles in multicellular organisms: diploid-dominant, in which the multicellular diploid phase is the most obvious life stage (and there is no multicellular haploid phase), every bit with well-nigh animals including humans; haploid-dominant, in which the multicellular haploid stage is the most obvious life stage (and in that location is no multicellular diploid phase), as with all fungi and some algae; and alternation of generations, in which the two stages, haploid and diploid, are credible to one degree or another depending on the group, as with plants and some algae.
Nearly all animals employ a diploid-dominant life-cycle strategy in which the only haploid cells produced by the organism are the gametes. The gametes are produced from diploid germ cells, a special jail cell line that merely produces gametes. One time the haploid gametes are formed, they lose the ability to divide again. There is no multicellular haploid life stage. Fertilization occurs with the fusion of two gametes, usually from unlike individuals, restoring the diploid land (Figure 7.two a).
If a mutation occurs so that a fungus is no longer able to produce a minus mating type, will it still be able to reproduce?
Most fungi and algae employ a life-cycle strategy in which the multicellular "body" of the organism is haploid. During sexual reproduction, specialized haploid cells from two individuals join to form a diploid zygote. The zygote immediately undergoes meiosis to grade four haploid cells called spores (Effigy 7.2 b).
The 3rd life-cycle type, employed by some algae and all plants, is called alternation of generations. These species have both haploid and diploid multicellular organisms as role of their life wheel. The haploid multicellular plants are called gametophytes because they produce gametes. Meiosis is non involved in the production of gametes in this case, equally the organism that produces gametes is already haploid. Fertilization between the gametes forms a diploid zygote. The zygote volition undergo many rounds of mitosis and give ascension to a diploid multicellular plant called a sporophyte. Specialized cells of the sporophyte volition undergo meiosis and produce haploid spores. The spores will develop into the gametophytes (Figure 7. 2 c).
Section Summary
Well-nigh all eukaryotes undergo sexual reproduction. The variation introduced into the reproductive cells by meiosis appears to be ane of the advantages of sexual reproduction that has made it so successful. Meiosis and fertilization alternate in sexual life cycles. The process of meiosis produces genetically unique reproductive cells called gametes, which accept half the number of chromosomes every bit the parent cell. Fertilization, the fusion of haploid gametes from two individuals, restores the diploid condition. Thus, sexually reproducing organisms alternate betwixt haploid and diploid stages. However, the ways in which reproductive cells are produced and the timing between meiosis and fertilization vary greatly. There are three main categories of life cycles: diploid-dominant, demonstrated past most animals; haploid-dominant, demonstrated by all fungi and some algae; and alternation of generations, demonstrated past plants and some algae.
Glossary
alternation of generations: a life-cycle blazon in which the diploid and haploid stages alternate
diploid-dominant: a life-cycle type in which the multicellular diploid stage is prevalent
haploid-dominant: a life-bicycle type in which the multicellular haploid stage is prevalent
gametophyte: a multicellular haploid life-cycle stage that produces gametes
germ cell: a specialized jail cell that produces gametes, such equally eggs or sperm
life wheel: the sequence of events in the development of an organism and the production of cells that produce offspring
meiosis: a nuclear segmentation process that results in four haploid cells
sporophyte: a multicellular diploid life-cycle stage that produces spores
Footnotes
i Leigh Van Valen, "A new evolutionary law," Evolutionary Theory ane (1973): 1–thirty.
Source: https://opentextbc.ca/biology/chapter/7-1-sexual-reproduction/
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