II. 5. Value of crossing-over between linked genes mo5 and n in barley.
O. T. Tazhin, Biological Faculty, Department of Genetics, Kazakh State University, Alma-Ata, 480091, U.S.S.R.
The initial mutant (var. Revelatum from Mongolia), which spontaneously emerged in 1964, and its identical progenies contain four ovaries in each flower (one ovary is main and the remaining three ovaries have developed from stamens), as well as two greenish leaflets instead of lodicules. The multiovary character in the mutant florets is controlled by a recessive gene mo5 linked with the hull-less seed gene n (Tazhin 1971, 1980). One may observe a series of multiple alleles mol-mo4 accounting for a great number of ovaries in barley (Moh and Nilan 1953; Kamra 1966) but a linkage group has not been established for them as yet.
When crossing mutants with hulled seeded barleys, F1 always have normal hulled seeds and the F2 segregates into normal hulled seed and multiovarian in the ratio 3:1. In addition, single normal plants with hull-less seeds were observed.
Multiovarian plants are homozygous as to their recessive and linked genes, their genotype being mo5 mo5 nn. In florets of the multiovarian plants all anthers transformed to ovaries, therefore self-pollination and fertilization do not occur in them and the hull-less seed gene is not evident. Hulled seeded barleys to be crossed with the multiovarian mutants are homozygous for both dominant genes, their genotype being ++/++. Their F1 are heterozygous for both genes and have the genotype +mo5/+n.
Two types of non-crossover (mo5n and ++) and two types of crossover (mo5 + and + n) ovicells and spermatozoons are formed with equal frequency in flowers of F1 plants.
As a result of self-pollination and fertilization of non-crossover ovicells with non-crossover spermatozoons hulled caryopses form in flowers of F1, as a consequence non-crossover plants in the ratio 3 normal hulled occur in F2: 1 multiovarian by phenotype and 1++/++:2+mo5/+n:l mo5mo5nn by genotype.
Due to fertilization of crossover gametes with the non-crossover ones hulled caryopses form in florets of F1, as a consequence crossover plants in the ratio 4 normal hulled occur in F2 :2 normal hull-less seed: 2 multiovarian by phenotype and 2+ mo5/++:2++/+n:2+ mo5/nn: 2 mo5mo5/+n by genotype.
Owing to fertilization of crossover gametes of two types hulled caryopses form in flowers of F1, as a consequence crossover plants in the ratio 2 normal hulled occur in F2 :1 normal hull-less seed: 1 multiovarian by phenotype and 2+mo5/+n:l++/nn: lmo5mo5/++ by genotype.
Thus in the presence of crossing-over during the formation of female
and male gametes four phenotypic classes arise in F2. However, crossover
hulled plants are phenotypically not distinguishable from the non-crossover
hulled ones and crossover multiovarian plants are phenotypically not distinguishable
from the non-crossover multiovarian plants F2. As a resul F2 phenotypically
segregated into 2 classes of plants (normal hulled and multiovarian), but
only single hull-less seed (crossover) plants with the genotypes +mo5/nn
and ++/nn segregate. Therefore, the crossing-over percentage was
derived only with consideration of one class (i.e. hull-less seed barley)
using
the formula
where p - fraction of crossover plants, n - total number of plants,
c number of hull-less seed plants in F2. The value of crossing-over between
the linked genes mo5 and n in barley thus derived is given
in Table 1.
During 1970-1978 segregation of 1O hybrids (F2 multiovarian x hulled) has been studied. Normal two-rowed hulled barley var. Nutans was used as a male parental form. As to the total of combinations, hybrids F2 segregated into normal hulled and multiovarian in the ratio 3:1. Ten hybrids of F2 taken into account, 61 plants with hull-less seed were established within 9 years. On the linearity test the progeny of each plant out of 61 plants segregated into normal hull-less seed and multiovarian in the ratio 3:1. Thus, all hull-less seed plants occurring in F2 were the crossover ones with the genotype +mo5/nn. The poor infructescense of hybrid grains on artificial pollination (on the average 3.3%), their low field germination rate (below 70%) made it impossible to obtain a sufficient number of plants in F1 and F2 to establish crossover hull-less seed plants with the genotype ++/nn among F2.
As is seen from Table 1, the value of crossing-over between the genes mo5 and n as to one class (i.e., hull-less seed barley) varies from 1-3% depending upon years and crossing combinations. It varies even within one and the same combination as far as years are concerned. Thus, as to 3 combinations F2 multiovarian x Nutans 19180 the crossing-over value varies within 1.6 - 2.5% depending upon years. As to 4 combinations F2 multiovarian x Nutans 187 it varies within 1.0 - 2.62% which probably depends upon the peculiarities of pollinator varieties and F hybrid growth condition and development. It is impossible to obtain fuil information on the crossing-over value estimation considering only the hull-less seed barley. Most valid data on the value of crossing-over between the genes mo5 and n were obtained as a result of two combinations of analyzing crossing. In such case the multiovarian maternal plant forms one type of ovicells (mo5n) while the heterozygous male parental plant forms as to both genes, i.e. hybrid F1, forms 4 types of spermatozoons, two of which are the non-crossover types (mo5n and ++) and the other two are the crossover ones (mo5+ and +n). When artificially pollinated by pollen F1 hybrid grains (on the average 3.3%) with poor field germination (below 70%) form in flowers of multiovarian plants, giving rise to non-crossover parental plants in the progeny, their ratio being 1 normal hulled: 1 multiovarian and that of the crossover ones being 1 normal hull-less seed: 1 multiovarian. The crossover multiovarian plants with the genotype mo5 mo5/+n are phenotypically not distinguishable from the non-crossover multiovarian ones. Due to this fact the hybrids phenotypically segregate into 2 classes of plants, i.e. normal hulled and multiovarian in the ratio 1:1 followed by segregating of normal hull-less seed (crossover) plants with the genotype +mo5/nn. Data on two combinations of crosses are given in Table 2.
As to the combination multiovarian x (F1 multiovarian x Nutans 187) the fraction of crossover hull-less seed plants is 2.01%. As far as the combination multiovarian x (F1 multiovarian x Nutans 19180) is concerned, it is 1.80%, the average being (2.01 + 1.80) 2 = 1.90%
It should be noted that the occurrence of multiovarian plants with the genotype mo5mo5/+n that are not phenotypically established is equal as compared with the crossover hull-less seed ones in the ratio 1:1. Thus, the fraction of the crossover hull-less seed plants class multiplied by 2 gives 1.90 x 2 = 3.80%. Consequently, the value of crossing-over between the linked genes mo5 and n in barley averages 3.8%.
References:
Kamra, P. 1966. Genetic control of the development of floral organs in Hordeum vulgare. Mechanism of mutation and inducing factors. Proceedings of Symposium in Prague, August 9-11, 1965, pp. 213-215.
Moh, C. C. and R. A. Nilan. 1953. Multi-ovary in barley. J. Hered. 44:183-184.
Tazhin, 0. T. 1971. Inheritance of the multiovarian characteristic by barley. In collection "Biological Sciences", Kazakh State University Press, Alma-Ata, pp. 69-72.
Tazhin, 0. T. 1980. The linkage of the genes mo5 and n in barley. Barley Genet. Newsl. 10:69-72.
