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Question

Organisms come into sexual maturity at different times.

a. temporal isolation

b. hybrid inviability

c. hybrid infertility

d. hybrid vigor

e. gametic incompatibility

Answer 1

a:
Temporal isolation: occurs because species mate at different time.
Example:
1: Different species of plants frequently have different flowering seasons.
2: Closely related species of fireflies frequently mate at different times of night.
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OR
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Populations may mate or flower at different seasons or different times of day. Three tropical orchid species of the genus Dendrobium each flower for a single day; the flowers open at dawn and wither by nightfall. Flowering occurs in response to certain meteorological stimuli, such as a sudden storm on a hot day. The same stimulus acts on all three species, but the lapse between the stimulus and flowering is 8 days in one species, 9 in another, and 10 or 11 in the third. Interspecific fertilization is impossible because, at the time the flowers of one species open, those of the other species have already withered or have not yet matured.

A peculiar form of temporal isolation exists between pairs of closely related species of cicadas, in which one species of each pair emerges every 13 years, the other every 17 years. The two species of a pair may be sympatric (live in the same territory), but they have an opportunity to form hybrids only once every 221 (or 13 ´ 17) years.
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B
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HYBRID INVIABILITY

Occasionally, prezygotic mechanisms are absent or break down so that interspecific zygotes (fertilized eggs) are formed. These zygotes, however, often fail to develop into mature individuals. The hybrid embryos of sheep and goats, for example, die in the early developmental stages before birth. Hybrid inviability is common in plants, whose hybrid seeds often fail to germinate or die shortly after germination.
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C
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HYBRID INFERTILITY

The evolution of hybrid infertility: perpetual coevolution between gender-specific and sexually antagonistic genes

new hypothesis is proposed for the rapid evolution of postzygotic reproductive isolation via hybrid infertility. The hypothesis is motivated by two lines of experimental research from Drosophila melanogaster that demonstrate that sexually antagonistic fitness variation is abundant and that epistatic fitness variation on the Y chromosome is common. The hypothesis states that the expression of sexually antagonistic genes leads to a 'gender-load' in each sex. In response, gender-limited reproductive genes are selected to ameliorate, through pleiotropy, the expression of sexually antagonistic genes. Chronic coevolution between gender-limited genes and gender-unlimited sexually an-tagonistic genes causes rapid divergence of reproductive proteins among allopatric populations, ultimately leading to hybrid infertility.

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D
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HYBRID VIGOR

Heterosis is a term used in genetics and selective breeding. The term heterosis, also known as hybrid vigor (or hybrid vigour) or outbreeding enhancement, describes the increased strength of different characteristics in hybrids; the possibility to obtain a "better" individual by combining the virtues of its parents.


Heterosis is often the opposite process of inbreeding depression, which increases homozygosity. Although it is believed that heterosis is the action of many genes of small effect, whereas inbreeding depression is the action of a few genes of large effect. The term often causes controversy, particularly in terms of the selective breeding of domestic animals, because it is sometimes believed that all crossbred plants or animals are better than their parents; this is not necessarily true. Rather, when a hybrid is seen to be superior to its parents, this is known as hybrid vigor. It may also happen that a hybrid inherits such different traits from their parents that make them unfit for survival. This is known as outbreeding depression, typical examples of which are crosses between wild and hatchery fish that have incompatible adaptations. Heterosis can be classified into mid-parent heterosis, in which the hybrid shows increased strength which is greater than the average of both parents, and best-parent heterosis, in which the hybrid's increased strength is greater than that of the strongest parent. Mid-parent heterosis is more common in nature, and it is easier to explain (by mechanism of gene dominance; see below).

Genetic basis of heterosis
Two leading hypotheses explain the genetic basis for fitness advantage in heterosis.

The overdominance hypothesis implies that the combination of divergent alleles at a particular locus will result in a higher fitness in the heterozygote than in the homozygote. Take the example of parasite resistance controlled by gene A, with two alleles A and a. The heterozygous individual will then be able to express a broader array of parasite resistance alleles and thus resist a broader array of parasites. The homozygous individual, on the other hand, will only express one allele of gene A (either A or a) and therefore will not resist as many parasites as the heterozygote.

The second hypothesis involves avoidance of deleterious recessive genes (also called the general dominance hypothesis), such that heterozygous individuals will express less deleterious recessive alleles than its homozygous counterpart.

The two hypotheses will have different consequences on the gene expression profile of the individuals. If over-dominance is the main cause for the fitness advantages of heterosis, then there should be an over-expression of certain genes in the heterozygous offspring compared to the homozygous parents. On the other hand, if avoidance of deleterious recessive genes is the cause, then there should be fewer genes that are under-expressed in the heterozygous offspring compared to the parents. Furthermore, for any given gene, the expression should be comparable to the one observed in the best of the two parents.


Hybrid corn
Nearly all the field corn now grown in the United States and most other developed nations is hybrid corn. Modern corn hybrids substantially outyield conventional cultivars and respond better to fertilization.

Heterosis in maize was first demonstrated in the early 20th century by George H. Shull and Edward M. East. They showed that crosses of inbred lines made from a Southern dent and a Northern flint, respectively, showed substantial heterosis and outyielded conventional cultivars of that era. However, at that time such hybrids could not be economically made on a large scale for use by farmers. Donald F. Jones at the Connecticut Agricultural Experiment Station, New Haven invented the first practical method of producing a high-yielding hybrid maize in 1914-1917. Jones' method produced a double-cross hybrid, which requires two crossing steps working from four distinct original inbred lines. Later work by corn breeders produced inbred lines with sufficient vigor for practical production of a commercial hybrid in a single step, the single-cross hybrids. Single-cross hybrids are made from just two original parent inbreds. They are generally more vigorous and also more uniform than the earlier double-cross hybrids.
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E
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GAMETIC INCOMPATIBILITY
if you mean
Genetic incompatibility ( we discuss both)

Genetic incompatibility
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Genetic incompatibility means that a plant cannot produce a zygote with its own pollen. There are two types, sporophytic and gametophytic. If the pollen is the same allele as that of the stigma, ten mating will not be successful.

AND IF YOU MEAN

GAMETIC INCOMPATIBILITY
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Quantitative Analysis of Gametic Incompatibility Between Closely Related Species of Neotropical Sea Urchins

Species of the sea urchin genus Echinometra found on the two coasts of Panamá are recently diverged and only partially isolated by incomplete barriers to interspecific fertilization. This study confirms previous work that revealed incompatibility between the eggs of the Atlantic E. lucunter and the sperm of the other two neotropical species, whereas eggs of its sympatric congener E. viridis and allopatric E. vanbrunti are largely compatible with heterospecific sperm. Here we quantify fertilization using a range of sperm dilutions. We demonstrate a much stronger block to cross-species fertilization of E. lucunter eggs than was previously shown at fixed sperm concentrations, and mild incompatibility of the other two species’ eggs where previous crosses between species were not distinguishable from within-species controls. Additionally, we present evidence for intraspecific variation in egg receptivity towards heterospecific sperm. Our findings here again discount the "reinforcement model" as a viable explanation for the pattern of prezygotic isolation. Gamete incompatibility in these Echinometra has appeared recently—within the last 1.5 million years—but is weaker in sympatry than in allopatry. Accidents of history may help explain why incompatibility of eggs emerged in one species and not in others. Compensatory sexual selection on sperm in this species could follow, and promote divergence of proteins mediating sperm-egg recognition.



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Answer 2

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RE:
Help with Bio"?
Organisms come into sexual maturity at different times.

a. temporal isolation

b. hybrid inviability

c. hybrid infertility

d. hybrid vigor

e. gametic incompatibility

Answer 3

DO YOUR OWN HOMEWORK.

I'm not going to help you take the place of someone smarter than you in a higher level course with a quota.

Answer 4

bio

Answer 5

belongs in the homework section