Hunt Institute Archives Text Discovery Platform

Page transcription

Biosynthetic VII

Species hybrids

I have mentioned previously that although plants with different chromosome numbers may be crossed, they then hybrids are actually sterile, so that their genes cannot be mixed or recombined. With other words, chromosomes with differences may allow crossability but not meiosis. Chromosome numbers vary from 2 = 2 in the Ascaris, which is more intestinal worm? The horse, (2 = 4, the Equine Hyalopogon prorius), to 2 = 1200, in the Indian fox Cynipiform reticulata. Nevertheless, there are many examples of such animals and plants having the same chromosome number species, that it is unavoidable that many unrelated animals and plants have the same chromosome number. The shape of the chromosomes and their size differs considerably. For example, the Scirpus albus, which are just visible in the light microscope since they are only 0.2 per log, to those of lilies which we can see with the naked eye! We distinguish chromosomes by their size but more frequently on basis of their morphology, or karyotype (Cherutsky 1932). Normally by aid of the localization of their centromere. If the centromere is situated in the middle of the chromosome, the chromosome is metacentric. If the centromere is situated towards one end of the chromosome, the chromosome is submetacentric, subtelocentric, acrocentric (micro-sized), telocentric (lengthy at the end), or ringed. Also said that the centromere is median, submedian, subterminal, terminal. Other secondary structures occur in the chromosome series; especially when satellites occur, connected with the junction of nucleoli, which are dense, spherical bodies rich in RNA and visible only wrongly in the interphase.

Even if different species have the same chromosome number and are related, they rarely cross if their chromosome morphology is not similar. And one geologist's evidence reported in medieval times has not been experimentally verified, e.g., the report of the creation of the ostrich from a hybrid between a giraffe and a camel! In animals, hybridization is often prevented because the individuals of different species do not wish to pair with each other. In Drosophila this has been studied in detail by bringing together females of one species with males belonging in part to the same species and in part to numerous closely related species. The females almost always prefer their own males. Hybridization is also prevented by the fact that the sexual organs in the two species are so differently constructed that pairing is excluded.

In flowering plants, species crosses are often prevented from the outset. The fact that pollen from one species is unable to germinate on the stigmas of the other. In other cases, the pollen may germinate, but pollen tube growth is so slow that fertilization and the place. In certain instances, however, this difficulty may be experimentally overcome by stimulating pollen tube growth through moderate X-ray doses or by repeated pollinations.

If the egg is fertilized and a hybrid embryo is formed, the embryo may degenerate during its development, resulting in an inviable seed. In several cases of this kind it has been found that embryo death is not caused by an incompatible embryo constitution, but by a physiological disharmony between the embryo and the mother plant. In other words, the mother is a "bad mother" for the hybrid seed. This has been demonstrated by excising the embryo from the ovule, followed by culture of the excised embryo on an artificial nutrient medium. The technique for artificial embryo culture has been gradually improved, and it is generally possible to raise a considerable number of species hybrids that otherwise would not have survived.