Desynapsis in diploid and tetraploid clones of ryegrass

Mutant genes affecting higher plant meiosis

That meiosis is conditioned by a large number of genes majority of which are present in a dominant state, is evidenced by the detection of numerous monogenic recessive mutant genes which affect the premeiotic, meiotic and post-meiotic course of events. These genes are site- and stage-specific, and a few are sex specific. Of these, the most prevalent are the mutant genes affecting male meiosis and causing male sterility (ms genes) and those inhibiting synapsis and chiasma formation (synaptic genes) and leading to gametic sterility. Majority of the mutant genes affect the entire chromosomal complement but a few influence only specific chromosomes of a complement so that the chromosomes behave differentially within a genome of the same species. Some mutant genes alter chromosome form and function, others modify integrity, degree of spiralization, movement and migration of chromosomes. Their cytogenetic behaviour, genetic significance and breeding utility are described and discussed.

Desynapsis in Coix lacryma-jobi caused by genotype-environment-interaction

Desynapsis leading to total sterility was observed in one plant of Job's tears, Coix lacrymajobi. L., during summer months, while the other neighbouring plants were quite normal. Pachytene chromosome pairing in the desynaptic was apparently normal but a diakinesis and metaphase I several univalents were found. Perennial suckers of this plant in the rainy season showed regular meiosis and good fertility. It is probable that desynapsis was caused by an interaction of genotype and environment, in which the desynaptic gene became activated by the rapid increase in summer temperatures whilst its effect on chromosome synapsis was completely lost in the cooler periods.

Trisomics and aneuploids of ryegrass

Primary trisomics (2 n + 1 = 15), double trisomics (2 n + 1 + 1 = 16) and aneuploids with 24 to 30 chromosomes, as well as a diploid and tetraploids, were found in the progeny of a hypertriploid (2 n = 22) plant of perennial ryegrass, Lolium perenne L. Trisomics and double trisomics differed in their mean chromosome association, chiasma number and spike morphology. A few aneuploids and tetraploids had reciprocal translocations. The diploid, primary trisomics and tetraploids were more fertile than the double trisomics and aneuploids. Most trisomics and aneuploids were probably produced through female transmission. One double trisomic had a high univalent number, a low chiasma number and loose chromosome coiling. Both the extra chromosomes carried secondary constrictions. The gene for desynapsis might be located on one of these chromosomes.

Chromosome behavior in a hypertriploid plant of ryegrass

A hypertriploid (2n = 22) was detected in the progeny of a desynaptic diploid (2n = 14) plant of perennial ryegrass,Lolium perenne L. The hypertriploid did not differ in morphology from its maternal-sib diploids, but showed larger stomata and pollen. The microsporocytes showed a mean chromosome association of 4.3 I + 3.4 II + 3.2 III + 0.3 IV at metaphase I with a mean chiasma number of 14.4 per cell. The 33 types of observed chromosome configurations could be explained by assuming that the plant was trisomie for 6 chromosomes and tetrasomic for one chromosome. Karyotype analysis confirmed the above assumption and revealed tetrasomy of chromosome number VI. However, two chromosome associations, 1I + 4 II + 3 III + IIV and 2 I + 5 II + 2 III + 1 IV suggested the presence of displaced duplications within the genome of ryegrass. The tetrasomic chromosome formed mostly a quadrivalent, which often broke down to form 2 II or 1 I + 1 III and rarely 2 I + 1 II or 4 univalents. Most of the univalents arose from the trisomic chromosomes and divided precociously at anaphase 1, producing diads and tetrads with unequal chromosome numbers. The plant was highly sterile and set no seed on controlled crossing.