In interspecific sterility, the failure in mating occurs because of inability of the sperm to reach the egg in animals and the pollen to reach ovules in plants. In plailts interspecific crosses usually result in the non-growing of the pollen tubes or the slowing down of the growth. If pollen from another species is transferred to a plant along with the pollen from conspecific individuals, the growth of the pollen tube of the latter is much faster than the former and all the fertilisation is conspecific.
In certain cases of interspecific crosses the pollen tube begins to grow but then bursts ensuring that no fertilisation occurs. Such an event occurs when the chromosome number of the inale parent is higher than female parent. For instance, three specie of tobacco plants are known to occur: Nicotiana tabacum, N, sylvestris and N. tomentosa. N, tabacutn has 48 chromosomes and the other two species have 24 each.
Probably N. tabacum is a tetraploid produced by a cross between the other two species. A cross between either of the two species and N. tabacum is successful only if the latter is used as a female parent. In such a cross the style tissues have 48 chromosomes and the pollen tube has only 12 chromosome, giving a ratio of 4:l. When the cross is between N. sylvestris and N. tomentosa, the style tissue of both the species has 24 chromosomes and the pollen tube has 12 chromosomes giving a 2:l ratio. But if N. tabacum were to be a male parent, and the either of the other two species a female, then the style tissue has 24 cl~romosomes and the pollen tube has 24 chromosomes giving 1:l ratio. It is only under these circumstances, that is when the ratio is close to 1:l the bursting of pollen tube occurs. It is assumed that a high osmotic pressure in the pollen tube causes it to burst and that this trait is controlled by a gene. Essentially a genetically coupled physiological mechanism prevents interspecific crosses in the tobacco plant.