The reason for hybrid sterility is still in the analysis stage, but some facts have been noted on the subject. It appears that sterility is caused from the X chromosome. This sterility affects the male in most species who have the inability to form functional gametes. This is caused due to a disturbance when the sex cell is developing. Hybrid sterility is found in both animal and plant life. Research is being conducted on both levels to determine if the two may be linked.
Breeding two animals that are not of the same breed but of the same species will produce a hybrid. For instance, breeding a female horse with a male donkey results in the hybrid known as a mule. The male mule is sterile and will not be able to reproduce. If a female hybrid is born, in most cases, she too will be sterile and unable to reproduce.
In mice of different subspecies, a study was conducted by the Transdisciplinary Research Integration Center in Tokyo, Japan. In this study of hybrid mice, it was found that the head of the sperm was not formed correctly; therefore, it was unable to penetrate the egg in order to allow fertilization. This malformation of the sperm head can be directly related to the genetic incompatibility of the sex-defining chromosomes.
There are results that prove, in some species, that the first-born from the first interbreeding will be fertile. The males may have a low sperm count, but are considered fertile. Hybrid sterility is noted upon breeding these hybrids. This generation has the inability to form the functional gametes due to incompatible genetic constitution of the hybrid.
The studies into the reason for hybrid sterility can be dated back to before 1861. Darwin wrote extensively on the subject of hybridism. In Darwin's experiments he used flowering plants to prove his points on hybrid sterility. It had been thought by many that sterility came from the plant itself; in order to keep the lineage pure, the plant would not accept fertilization from another plant not of like kind. In this case, and under certain circumstances, this would eventually render the plant extinct.
Darwin believed that the will to survive was strong and that each plant had the sexual powers to determine its outcome. Although a plant may not be receptive to cross-pollination from a plant that would result in sterility, it would indeed be receptive to cross-pollination from a stronger plant that--although it may change the plant--would secure survival. By the end of this study, Darwin's conclusion was that cross and hybrid sterility had arisen incidentally.
Although studies still continue on the reasons for hybrid sterility, it is important to note that Darwin may not have been far off on his research. In 1988 a female mule and a female hinny (offspring of a male horse and a female donkey) were both found to be fertile, and both had female offspring. The fillies both show unique hybrid chromosomes that are not only different from their parents, but they are different from each other. This could lead to proof that perhaps hybrid sterility is a selective process.