Which of the following statements best describes why prevention of cross-species

Question

Which of the following statements best describes why prevention of cross-species fertilization is different between sea urchins and fruit flies?

A. Fruit flies need more regulation of specifies-specific sperm and egg interactions compared to sea urchins since fruit fly fertilization occurs externally.

B. Fruit flies need less regulation of species-specific sperm and egg interactions compared to sea urchins since fruit fly fertilization occurs externally.

C. Fruit flies need more regulation of species-specific sperm and egg interactions compared to sea urchins since fruit fly fertilization occurs internally.

D. Fruit flies need less regulation of species-specific sperm and egg interactions compared to sea urchins since fruit fly fertilization occurs internally.

E. None of the statements above account for the differences in preventing cross-species fertilization.

The syncytial blastoderm and the cellular blastoderm differ in that

A. maternal components diffuse freely in the cellular blastoderm, but not in the syncytial blastoderm.

B. maternal components diffuse freely in the syncytial blastoderm, but not in the cellular blastoderm.

C. the cellular membranes of the syncytial blastoderm prevent cell-cell communication within the developing embryo.

D. the cellular membranes of the cellular blastoderm prevent cell-cell communication within the developing embryo.

E. None of the above.

Explanation / Answer

1. Fertilization also known as syngamy is the process of fusion of male and female gametes to form new organism of the same species. In animal species the process of fertilization can take place within the body of female or outside the female body.

External fertilization - This process usually occurs in aquatic environments where both sperms and eggs are released in water. The external fertilization usually occurs during spawning when one or many females lay eggs and male release sperm in the same area and at the same time. The temperature of water and the length of daylight can also effect the release of eggs and sperms. The examples of external fertilization are fishes, crustaceans, molluscs, echinoderms(sea urchins).

Internal fertilization is prevalent in land based animals although some aquatic organisms can also show internal fertilization. There are three ways of internal fertilization:-

In Drosophila oviparity is seen.

In sea urchins the fertilization is external and species-specific sperm attraction is reported in them. The meeting of sperm and egg in marine environment is not a simple matter as the environment can be small or very big like in oceans. Another difficulty is that the environment is shared by other species which also release their sperms and eggs in the same environment.

The sperm is attracted by the eggs by chemotaxis which is the process of following a path or gradient of a chemical released by the egg. The mechanism of chemotaxis is different among the different species. For example; the chemotactic molecule called resact has been isolated from the egg of sea urchin Arabica punctulata attracts the sperm of the same species.

Therefore the option D is correct- the fruit flies need less regulation of species-specific sperm and egg interactions to sea urchins since fruit fly fertilization occurs internally.

2. The formation of syncytial blastoderm takes place when the zygote nucleus divides through many mitotic divisions not followed by the cell division In Drosophilla 256 nuclei are produced by 8 mitotic divisions. These nuclei migrate to the periphery of the egg where again mitosis takes place but at a very slow rate. In the ninth mitotic division 5 nuclei migrate to the posterior pole of the embryo which get enclosed in cell membranes and form the pole cells. There are more mitotic divisions that take place at slow rates. The embryo formed is called syncytial blastoderm with all the nuclei in same cytoplasm.

          The cellular blastoderm is formed by cellularization of the nuclei present in syncytial bastoderm. This takes place by invagination of cell membranes between the nuclei which forms furrow canal. When furrow canal has passed the level of nuclei second level of cellularization takes place. In Drosophilla the cellular blastoderm consists of 6000 cells which are formed in 4 hours after fertilization.

Therefore the option B is correct- the maternal components diffuse freely in the syncytial blastoderm, but not in the cellular blastoderm.

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