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The term asynapsis was originally proposed by Randolph (1928) to describe the absence of normal chromosome pairing during first meiotic division. The meiotic mutant was first described by Beadle and McClintock (1928) in maize. They reported that the majority of the sporocytes at metaphase-I showed 20 univalents (I) and rarely 10 bivalents (II). Mutant plants showed variable pollen and ovule abortion. Beadle (1930) assigned the gene symbol as to this particular asynaptic mutant.

Table 4.5 Desynaptic Genes in Barley Desynaptic

Genes Origin

Chromosome Behavior (Range Bivalents)

Female

Fertility (%) Chromosomes Inheritance

des1 X-rays 7
II to 5 rod II + 4 I 45 1 3:1

des2 X-rays 7
II to 2 rod II + 10 I 1 3 3:1

des3 S 7
II to 4
II + 2 rod II + 2 I 1–33 — 3:1 des4 S 7
II to 3 rod II + 8 I 18 1 3:1 des5 S 7
II to 14 I 7 1 3:1 des6 S 7
II to 14 I 16 1 3:1 des7 S 7
II to 14 I 33 2 3:1 des8 S 7
II to 14 I 22 — 3:1 des9 S 7
II to 5 rod II + 4 I 90 — 3:1 des10 S 7
II to 3
II + 2 rod II + 4 I 60–80 — 3:1

des11 S 6
_{II + 1 rod II to 2 rod II} + 10 I 40 — 3:1 des12 S 7
II to 1 rod II + 12 I 35 — 3:1 des13 S 7
II to 4
II + 2 rod II + 2 I 20–30 — 3:1 des14 S 7
_{II to 1 rod II + 12 I} 35 — 3:1

des15 S 5
_{II + 2 rod II to 1 rod II} + 12 I

25 —

Sources: des1–8 from Hernandez-Soriano, J.M., Ramage, R.T., and Eslick, R.F., Barley Genet. Newsl., 3,

124–131, 1973; des 9–14 from Hernandez-Soriano, J.M. and Ramage, R.T., Barley Genet. Newsl., 4, 137–142, 1974; and des 15 from Hernandez-Soriano, J.M. and Ramage, R.T., Barley Genet. Newsl., 5, 113, 1975.

Table 4.6 Cytological Observation and Pollen Fertility of Sterile and Partially Sterile Lines from Two Ethyleneimine Treatments

Ethyleneimine Treatment Cytological Observation 1 h, 0.4% 3 h, 0.2% Total

Pollen Fertility Range (%)

Reciprocal translocations 39 22 61 1.6–80.0

Triploids 1 1 2 0.9–2.1

Desynaptic plants 1 1 2 2.1–22.1

Meiosis not studied 4 7 11 33.0–89.0

Normal meiosis 4 9 13 80–100

Normal meiosis 2 2 4 0.0–4.0

Total 51 42 93

Source: From Singh, R.J. and Ikehashi, H., Crop Sci., 21, 286–289, 1981. With

Asynaptic mutants do not exhibit normal pairing of homologous chromosomes at pachynema (failure to synapse in the first place), while failure to maintain association after the first synapsis is known as desynapsis. In asynapsis, most or all of the chromosomes remain univalents at diakinesis and metaphase-I (Table 4.2). This behavior indicates that the asynaptic mutant of maize described by Beadle (1930) is probably asynaptic rather than desynaptic. Miller (1963) examined as mutants cytologically and observed zero to nearly complete synapsis of homologous chromosomes at early pachynema and at metaphase-I. The range of bivalents was 0 to 9.95. In the desynaptic (dy) mutant of maize, bivalents are mostly loosely associated (Figure 4.2). The maize asynaptic (as) mutant also induces polyploid meiocytes, elongated and curved spindles, misdivision of univalents, and monod centromere and partial or complete failure of cytokinesis after the meiotic divisions (Miller, 1963).

Asynaptic mutants have been reported in several plants (Table 4.2). In Brassica campestris (2n

= 20), Stringam (1970) isolated three meiotic mutants. Mutants as and as3 displayed no chromo-

some pairing at pachynema, and no bivalents were recorded at metaphase-I. The as2 mutant showed a variable number of bivalents (up to four loosely paired) at diakinesis. Kitada and Omura (1984) identified one asynaptic (MM-19) and two desynaptic (MM-4, MM-16) mutants from N-methyl-

N-nitrosourea-treated rice seeds. In the MM-19 mutant, the homologous chromosomes lacked

complete synapsis at pachynema, resulting in 24I at metaphase-I. In contrast, mutants MM-4 and MM-16 showed variable chromosome pairing from zygonema to pachynema and displayed uni- valents and bivalents at metaphase-I.

Chromosome disjunction at anaphase-I to telophase-I is highly irregular in asynaptic mutants. The second division is essentially normal, but cells inherit chromosomal abnormalities resulting from the first meiotic division. Asynapsis produces chromosomally unbalanced male and female spores, resulting in a high level of pollen and ovule abortion. Gottschalk (1987) Figure 4.2 A microsporocyte at diakinesis from a desynaptic maize plant showing eight bivalents [One is nearly separated to univalents and two pairs of widely separated univalents. In one pair of uni- valents, there is an equationally separated distal knob (arrows), which indicates that a crossover has occurred between the knob and the kinetochore, so that in each univalent, there is a knob- carrying and knobless chromatid.] and four univalents. (From Maguire, M.P., Paredes, A.M., and Riess, R.W., Genome, 34, 879–887, 1991. With permission.)

found in x-ray-induced pea desynaptic mutants that ds genes influence microsporogenesis more strongly than megasporogenesis.

It has been demonstrated by electron microscopy (EM) that in asynaptic mutants, formation of the synaptonemal complex (SC) is blocked (La Cour and Wells, 1970; Golubovskaya and Mash- nenkov, 1976). La Cour and Wells (1970) examined two synaptic mutants of Triticum durum (2n

= 4x = 28) by light and electron microscopes. The light microscope showed suppression of zygotene

and pachytene pairing, and EM revealed the absence of SC. When SC is eliminated in the bivalents, the two lateral elements (cores) are set free (Figure 4.3). Recently, Maguire, Paredes, and Riess (1991) observed normal crossing over followed by failure of chiasma maintenance in a desynaptic mutant of maize. They examined normal and desynaptic stocks by EM and found statistically significant wider dimension of the SC central region and less twisting of the synapsed configuration

Figure 4.3 Asynaptic (as) nucleus at mid to late pachynema. Note the widespread lack of homologue pairing and short triple association. (Courtesy of M.P. Maguire.)

at pachynema in desynaptic mutants compared to normal. Chromosomes undergo desynapsis after pachynema to diakinesis, and by metaphase-I, the desynapsis is completed. The SC is apparently rapidly disintegrated following pachynema (Maguire, personal communication, 1992). A reduction of chiasma frequency or complete failure of chiasma formation occurs at diplonema to metaphase- I, resulting in various frequencies of univalents and bivalents. The action of desynaptic genes differs among sporocytes as well as among plants.

Because pachytene chromosome analysis is not feasible for a large number of plant species, the action of desynaptic genes is often ascertained on the basis of studies of diakinesis and metaphase-I. Chromosome associations at these later stages of meiosis appear to correlate well with the amount of pachytene chromosome synapsis. The degree of desynapsis is reflected by the number of bivalents at metaphase-I and the frequency of chiasmata per cell. Chiasmata are normally not randomly distributed among cells, chromosomes, and bivalents. They also vary between gen- otypes and between and within cells (Rees, 1961; Jones, 1967, 1974). Chiasmata in desynaptic plants are mostly terminal at metaphase-I and are rarely interstitial (Li, Pao, and Li, 1945).

Prakken (1943) classified desynaptic mutants depending upon their expressivity: weakly desyn- aptic (several univalents), intermediate desynaptic (many univalents), and completely desynaptic (exclusively univalents and rarely any bivalents).

Bivalents move to the equatorial plate at metaphase-I, while univalents tend to be distributed at random in the cytoplasm. The number of univalents varies within different microsporocytes in the same plant. This suggests that within a chromosome complement of a species, there may be differences among the different chromosomes concerning their requirements for the initiation of pairing (Rees, 1958; Swaminathan and Murty, 1959; Koduru and Rao, 1981).

Disjunction of bivalents at anaphase-I is usually normal. Univalents sometimes move to the poles at random without dividing, while in other cases, they divide equationally (Soost, 1951; Miller, 1963). Univalents that fail to move to either pole remain as laggards at the equatorial plate. At telephase-I, those chromosomes that reach the poles organize dyad nuclei, while laggards often form micronuclei. The second meiotic division is essentially normal, and irregularities are restricted to the first meiotic division. As a consequence of meiotic irregularities in asynaptic and desynaptic mutants, chromosomally unbalanced male and female microspores and megaspores are generated, resulting in reduced pollen and ovule viability.