Genetic studies in dioeciousmelandrium. II

The process of sex determination leads to the formation of the sex organs. Formerly two different groups of sex-determining genes were distinguished: the sex-promoting genes causing the actual development of the sex organs, and sex-deciding genes, which decide whether female-promoting genes, or male-promoting genes, or both come into action, thus giving rise to female flowers, male and hermaphrodite ones respectively. Obsrvations on our own material and data from the literature lead us to the conclusion that no such sharp distinction could be made. In this respect especially the hypothesis ofHeslop-Harrison (1957) on the role of auxin in sex determinations is of interest. It is argued that the sex-determing genes together provoke in flower primordia a certain net auxin activity that determines the development of the sex organs. A genothype that realizes a relatively high net auxin activity will favour the develoment of pistils and will work against the formation of stamina. A genotype that causes a relatively low auxin activity has the opposite effect. Changes of distinctly different sex-determining genes might have a similar effect on sex-expression. For example changes of genes in the Y-chromosome of an XY plant may cause the development of a pistil in the flowers of an XY plant and simultaneously work against the formation of stamina, as is demonstrated by some mutations in ourMelandrium material. In one mutation cytologically no aberration of the Y-chromosome was observed. The Y-chromosome of the other mutations appered to lack a part of their non-homologous arms. We observed the same phenotypical change after autosomal selection started in threeM. dioicum populations. A new feature of the distal part of the non-homologous arm of the Y-chromosome showed up in our hermaphrodite gerontogones. This part appeared to be indispensable for the functioning of both female and male gametes. In the discussion the formation of unisexual flowers in monoecious species and in dioecious ones was regarded to be fully comparable. In both instances auxin was assumed to play a dominant part in the differentiation of the flowers. We discussed the possible role of auxin in the regulating mechanism of protein synthesis. The origin of dioecism was explaned in terms of building up an uneven distribution of + genes (increasing the net auxin activity) and-genes (decreasing the net auxin activity) in a non-homologous pair of chromosomes. It is suggested that apomictic plants are like female plants characterized by a relatively high auxin activity in the flower primordia. We assumed therefore apomixis and dioecism to be by their origin related phenomena. With regard to the discrepancy in occurrence of dioecism and polyploidy in the plant kingdom and in the animal kingdom, it is argued that in plant species polyploidisation interferes with the building up of the balanced dioecious system, a situation not met with in the animal kingdom, because of the unfavourable aspects of polyploidy in animals.     

Genetic studies in dioecious melandrium. I

Sex-linked and sex-influenced inheritance are of interest because of their relation to the still intriguing problem of sex detrmination. Genes involved in the formation of the sex organs are regarded to be sex-determining genes. These genes may be present in all chromosomes including the sex-chromosomes. Other genes present in the sex-chromosomes, but not involved in sex determination, are the sex-linked genes. A mutation for narrow leaves we came across in ourM. ablum material is regarded as a case of sex linkage. Also the certation effect observed inM. album andM. dioicum must have been caused by genes on the sex-chromosomes. In both cases, however, it is not altogether unikekely, that the genes, regarded as sex-linked ones, actually take in the process of sex-determination.Sex-determining genes might influence the effect of other genes, that are therefore called sex-influenced genes. We observed a number of such sex-influenced characters inMelandrium.InM. album, female plants are, on the whole, larger than male plants, having larger stems and leaves. The petals, however, are larger in male plants, except in families with very broad petals. The leaves and petals are narrower in female plants than in male ones, except in families with very broad leaves and families with broad petals, where the difference in shape was no longer present. Usually, slightly more anthocyanin is formed in male plants than in females both in petals and the green parts. More glandular hairs were observed on male plants than on female ones.Insofar the observations were made inM. dioicum the same results were obtained.We regard these phenomena to be an expression of the different physiological conditions in female and in male plants, these conditions being provoked by the sex-determinging genes and more favourable for vegetative growth in female than in male plants.     

Genetic linkage studies of human neurodegenerative disorders.

Recombinant DNA technology has the ability to delineate the causes of several neurodegenerative disorders. Genetic linkage studies have been used successfully to localize gene defects and it is likely that in the near future the exact loci will be determined.     

Genetic studies of the Syrian hamster. X. Rex.

The rex mutant has its most obvious effect on coat structure but it also reduces 21-day body weight by about 14 per cent. There is no apparent effect on viability nor on the fecundity of the more robust rex females. Extensive tests for linkage with the genes b, Ba, cd, Ds, e l, Lg, ru, Sa and Wh have proved to be negative. A less extensive test with s was also negative.     

Genetic studies of a muscular dystrophy of mink.

Analysis of breeding data revealed that the muscular dystrophy trait in a pedigree of mink is transmitted in an autosomal recessive manner. Variation in skeletal muscle fiber diameter size is the most pronounced and consistent change in the dystrophic mink. Other changes include centralization of nuclei, degenerative change, increase in endomysial and perimysial connective tissue, and regenerative attempts. These changes are not present in known heterozygotes.