Inversion, which has chromosome rearrangement with reverse order of genes in a certain segment, classified into two types, i.e., pericentric inversion possessing centromere within inverted region and paracentric inversion not including centromere within inverted region. Crossing over in the inverted region produces deficiency-duplication chromosomes, and the resulted gametes are lethal. Thus, if inversion is occurring in sufficiently long region on chromosome, most of the crossing over of corresponding chromosome results in lethal gametes in the inversion heterozygote. Thus few gametes receiving a product of crossing over on the subject chromosome are transmitted to the progeny. By using this genetic phenomenon, inversion has been used as the mapping tool of genes of unknown linkage groups and as the isolation tool of a specific chromosome in diploid organisms.

In the heterozygote of a paracentric inversion, we can find a bridge and a fragment at Al of meiosis. Because we cannot find any aberration of meiotic division in the heterozygous pericentric inversion, we can only define pericentric inversions by observing loop formation at pachytene of meiosis. All inversions except one (Linde- Laursen, 1983) reported were paracentric (Smith, 1941; Holm, 1960; Powell and Nilan, 1968; Ekberg, 1969; Yu and Hockett, 1979). Moreover, the induction frequency of inversion was less than that of reciprocal translocation (Ekberg, 1969). Thus, an effective method of developing pericentric inversions is not proposed yet. However, a genetic male sterile line that was developed by Falk and Kasha (1982), made it possible to induce and select inversions on specific chromosomes except chromosome 6, on which the male sterile gene (msg6) is located by gamma-ray irradiation.

The procedure is as follows: 1) Seeds from heterozygote for male sterile gene of Falk and Kasha (1982) are irradiated by gamma-rays. 2) Multiple marker line of seedling characters, for example f9-g13 on chromosome 4 (Takahashi, et al., 1969), and irradiated lines after presowing selection are planted by row alternatively. 3) The M₂ bulk population is constructed by one seed from one spike of irradiated M₁ plants. 4) In M₃ generation, highly significant lines deviated from the segregation ratio of 9 F9G13 : 3 F9g13 : 3 f9G13 : 1 f9g13 are selected. 5) Meiotic observation at pachytene and AI of these selected lines are carried out. In this generation, most of the plump seeds are fertile, because they are heterozygotes for male sterile gene or normal genotype. Consequently, not only pericentric but also paracentric inversions on chromosome 4 should be frequently obtained. If we use a marker line, of which genes located on chromosome 6, resulted inverted chromosome has always male sterile gene, msg6. Therefore, inversion homozygote obtained in the next generation should be homozygous for male sterile gene, then produce no seed. In addition various gene mutations affected on segregation ratio may be also obtained.

References:

Ekberg, 1. 1969. Different types of sterility induced in barley by ionizing radiations and chemical mutagens. Hereditas 63:257-378.

Falk, D. E. and Kasha, K. J. 1982. Registration of a shrunken endosperm, male-sterile germplasm to facilitate hybridization in barley. Crop Sci. 22:450.

Holm, G. 1960. An inversion in barley. Hereditas 46:274- 278.

Linde-Lauresen, 1. 1983. A pericentric inversion in chromosome 3 of a translocation line of barley. BGN 13:54-55.

Powell, J. B. and Nilan, R. A. 1968. Evidence for spontaneous inversions in cultivated barley. Crop Sci 8:114-116.

Smith, L. 1941. An inversion, a reciprocal translocation, trisomics, and tetraploids in barley. J. Agr. Res. 63:741-750.

Takahashi, R., Hayashi, J., Konishi, T. and Moriya, I. 1969. Inheritance and linkage studies in barley. V. Locating of seven new mutant genes. Ber. Ohara Inst. Landw. Biol., Okayama Univ. 15:147-168.

Yu, C. W. and Hockett, E. A. 1979. Chromosomal behavior, breeding characteristics, and seed set of partially sterile barley, Hordeum vulgare L. Z. Pflanzenzuch. 82:133-148.