SYNAPTONEMAL COMPLEXES IN INSECTS

The synaptonemal complex (SC) is the key nuclear element formed in mei otic prophase I to join 2 homologous chromosomes at the pachytene biva lent. It is a highly conserved structure that is universally present i n eukaryotes. The SC is presented as a tripartite protein structure, w hich consists of 2 lateral elements and a central region. In insects, the central region is particularly distinct and highly ordered. This m ade it possible to describe the fine structure of the central region a nd propose a model of its architecture. Chromatid DNA is arranged in c hromatin loops extending radially from the SC. The loops appear to con sist of a basic chromatin fiber with a diameter of 20-30 nm. In many i nsect species, synaptonemal polycomplexes occur in postpachytene cells . They represent one of the possible ways of SC degradation. Another p rocess, which occurs beyond pachytene, is the formation of proteinaceo us chromatid axis, the silver-stained chromatid core. Based on results in insect models, the chromatid cores have been related to the struct ure and formation of the SC. Research on insect models significantly c ontributed to understanding individual steps of the SC formation and t emporal sequence of chromosome pairing. These include the formation of lateral elements of the SC, pairing initiation, interlocking of chrom osomes, and synapsis of homologous chromosomes. Attention is also give n to non-homologous pairing, including synaptic adjustment, correction of pairing, and pairing of sex chromosomes. In the next section, chia smatic and achiasmatic modes of meiosis are compared with respect to t he SC formation. In the chiasmatic mode, the SCs display recombination nodules that are believed to mediate the process of recombination. Th ese nodules were discovered in insects, and indirect evidence for thei r role comes from insects. Two different examples of achiasmatic meios is, occurring in the heterogametic sex of several insect orders, are g iven: one involves the SC formation, whereas in the other, SCs are abs ent. Finally, the potential of SC karyotyping for analysis of the inse ct genome is discussed. Copyright (C) 1996 Elsevier Science Ltd.