Question

Provide the biological explanation for why cross-species fertilization does not take place.

Compare and contrast embryo development in the frog and the chicken.

How might embryology help us understand the classi ca on of related organisms? (Taxonomy)

Answer 1

Cross species fertilization is not favoured among species in nature due to reproductive isolation mechansims that are operative in nature.

The "Biological species concept " defines species as basic fundamental unit where the species can interbreed freely among the same group but are reproductively isolated from other groups.

The isolating mechanisms that are operative in nature can be classified into two major groups they are:

A.  Pre-mating or prezygotic isolating mechanisms.

B. Post-mating post zygotic isolating mechanisms.

These are barriers which prevent interbreedg . They avoid it either before fertilization, as in prezygotic ones, or after fertilization has occurred as in postzygotic.Both the mechanisms are further classified into subtypes.

A.  Pre-mating or prezygotic  isolating mechanisms: These mechanisms are operative before mating occurs , the reason for the failure of mating or even after mating the formation of ga,metes doesnot occur due to

1.Temporal isolation : Individuals of different species do not mate because they are active at different times of day or in different seasons which prevents them copulating at the same time.

2. Ecological isolation : Individuals mate in their preferred habitat, and therefore do not meet individuals of other species with different ecological preferences.living beings from different areas cannot mate as they cannot possibly meet either.

3.Behavioral isolation : During the selection of a possible mate, individuals from different species may discard each other as they don’t have the same mating rituals thus only  Potential mates meet, but choose members of their own species.

4.Mechanical isolation : Different species may have different sex organs which are not compatible. Copulation is attempted, but transfer of sperm does not take place.

Post-mating post zygotic isolating mechanisms : These mechanisms are operative after mating. The following are the barriers that prevent the formation an viable off spring

1.Gametic incompatibility. Sperm transfer takes place, but egg is not fertilized. For fertilisation to occur, the sperm must reach the ovum. this happens usually when attract each other through chemical means, but the chemicals might vary from one species to another. In this case gametes won’t recognize each other and fertilization won’t take place.

2.Zygotic mortality. Egg is fertilized, but zygote does not develop, even if an offspring is produced from the mating of different species usually they are infertile as they generally have a random mixed number of chromosomes .So fertile off spring is not possible among different hybrids also.

3. Hybrid inviability. Hybrid embryo forms, but of reduced viability. the hybrid dies prematurely.

4.Hybrid sterility. Hybrid is viable, but resulting adult is sterile.

5.Hybrid breakdown. First generation (F1) hybrids are viable and fertile, but further hybrid generations (F2 and backcrosses) may be inviable or sterile. If the hybrid results to be fertile, the hybrid population might dissappear along time as from one generation to the next they may result weaker, less fertile, etc.