ASY1 is an Arabidopsis protein required for synapsis and crossover formation during meiosis. The chronology of meiotic recombination has been investigated in wild type and an asy1 mutant. We observe a delay between the appearance of chromatin-associated AtSPO11-1 foci and DNA double-strand break (DSB) formation, which occurs contemporaneously with chromosome axis formation and transition of ASY1 from chromatin-associated foci to a linear axis-associated signal. DSBs are formed independently of ASY1 in an AtSPO11-1-dependent manner. They are partially restored in Atspo11-1-3 using cisplatin, but their control appears abnormal. Axis morphogenesis is independent of ASY1, but axis structure may be compromised in asy1. Localization of the strand exchange proteins AtRAD51 and AtDMC1 to the chromatin occurs asynchronously shortly after DSB formation, with AtDMC1 localizing in advance of AtRAD51. In wild-type nuclei, both recombinases form numerous foci that persist for ∼12 h before gradually decreasing in number. In asy1, initial localization of AtDMC1 is normal, but declines abruptly such that interhomolog recombination is severely compromised. Limited ASY1-independent, DMC1-dependent interhomolog recombination remains, but appears restricted to subtelomeric sequences where the homologs are fortuitously in proximity. Thus, ASY1 plays a key role in coordinating the activity of the RecA homologs to create a bias in favor of interhomolog recombination.

Meiotic chiasmata were analysed in metaphase I pollen mother cells (PMCs) of wild-type Arabidopsis thaliana and in two meiotic mutants. Fluorescence in situ hybridisation (FISH) with 45S rDNA and 5S rDNA as probes was used to identify the five chromosome pairs. A wild-type chiasma frequency of 9.24 per cell was found, consistent with estimated genetic recombination values. Individual bivalent chiasma frequencies varied according to chromosome size; chromosome 1 had the highest mean chiasma frequency (2.14) while the short acrocentric chromosomes had the lowest frequencies (1.54 and 1.56). FISH analysis was extended to two meiotic mutants (asy1 and dsy1) having low residual bivalent and chiasma frequencies. Mutant dsy1 gave no indication of chromosome preference for residual bivalent formation; instead it showed a general reduction in bivalent and chiasma frequencies. In asy1, the longest chromosome (1) had the lowest bivalent frequency and chiasma frequency while the short acrocentric chromosome 2 had the highest frequencies. This chromosome pair may be preferentially involved in synapsis and chiasma formation because of their association with the nucleolus. However, other factors may be operating since the other acrocentric chromosome (4), with similar size and structure to chromosome 2, did not share these chiasma properties.

A surface-spreading synaptonemal complex (SC) technique was used to analyze spermatocytes and oocytes of turbot (Scophthalmus maximus) to visualize the process of chromosome synapsis. The total SC length was 205 +/- 12 microm in males and 172 +/- 29 microm in the only female analyzed. A representative SC karyotype of turbot was obtained. Each SC showed lateral elements of equal length. No bivalent exhibiting atypical synaptic behaviour that could be associated with heteromorphic sex chromosomes was observed, either in males or in the female. The DNA content of turbot was evaluated in eight individuals of both sexes by flow cytometry analysis. The 2C mean DNA content of turbot (1.308 +/- 0.009 pg/cell) was among the lowest observed within fishes. No statistical differences in DNA content were revealed between the sexes [Wilcoxon/Mann-Whitney test; P(W(x) = 0.243)]. The SC/DNA content ratio observed in turbot was the highest reported to date in bony fishes (Osteichthyes).

Meiotic behavior has been investigated in rye plants carrying a heterozygous ditelocentric substitution for chromosomes 1R and 3R. Such a chromosomal rearrangement replaces a biarmed chromosome with two telocentrics, a situation karyotypically comparable to a Robertsonian translocation. As in Robertsonian translocation heterozygotes, delayed pairing characterizes the centromeric region in the rye pachytene heteromorphic trivalents. In addition, telocentric axes appear to be frequently engaged in nonhomologous synapsis (e.g., fold-back loops and two-by-two heterosynapsis) at their centromeric ends. Unlike those in Robertsonian translocation heterozygotes, the synaptic abnormalities detected here cannot be due to lack of partner homology. Rather, the chromosomal rearrangement itself appears to interfere with normal synaptonemal complex (SC) progression. The synaptic irregularities, which often result in complex pachytene multivalents, seem to have no effect on chiasma formation, since the rearranged pairs behave properly at metaphase I. Finally, differences in both SC length and progression patterns between plants and animals could explain the trans configurations of telocentric centromeres in plants, whereas only cis configurations of acrocentric kinetochores have been described in animals.