MOST UV-INDUCED RECIPROCAL TRANSLOCATIONS IN SORDARIA MACROSPORA OCCUR IN OR NEAR CENTROMERE REGIONS

Abstract

In fungi, translocations can be identified and classified by the patterns of ascospore abortion in asci from crosses of rearrangement x normal sequence. Previous studies of UV-induced rearrangements in Sordaria macrospora revealed that a major class (called type III) appeared to be reciprocal translocations that were anomalous in producing an unexpected class of asci with four aborted ascospores in bbbbaaaa linear sequence (b = black; a = abortive). The present study shows that the anomalous type III rearrangements are, in fact, reciprocal translocations having both breakpoints within or adjacent to centromeres and that bbbbaaaa asci result from 3:1 disjunction from the translocation quadrivalent.—Electron microscopic observations of synaptonemal complexes enable centromeres to be visualized. Lengths of synaptonemal complexes lateral elements in translocation quadrivalents accurately reflect chromosome arm lengths, enabling breakpoints to be located reliably in centromere regions. All genetic data are consistent with the behavior expected of translocations with breakpoints at centromeres.—Two-thirds of the UV-induced reciprocal translocations are of this type. Certain centromere regions are involved preferentially. Among 73 type-III translocations, there were but 13 of the 21 possible chromosome combinations and 20 of the 42 possible combinations of chromosome arms.

Correlation between pairing initiation sites, recombination nodules and meiotic recombination in Sordaria macrospora.

“…All rescued the early equational separation of sister centromeres and the meiotic arrest of spo76, suggesting that they act at an earlier step in meiotic prophase I (HUYNH, LEBLON and ZICKLER 1986).…”

“…Most genetic markers used in this study were described previously (ZICKLER et al 1984;LEBLON, ZICKLER and LE-BILCOT 1986). The wild-type strain was isolated from one spore and all mutageneses were performed on this strain.…”