Description

Classical dominant intrachromosomal crossover suppressors in C. elegans (e.g. mnC1, sC1, qC1, mC6 [now mIn1]) have long been thought likely to be inversions. Prior to economical genome sequencing, proof of inversion depended on the isolation of crossover suppressor variants differentially marked with morphological mutations, construction of homozygous suppressor genotypes with different such markers in trans, and recombination analysis within and outside the putative inverted region (hIn1: Zetka and Rose, 1992; mIn1: Edgley and Riddle, 2001).

Here we report genome sequence data confirming that the crossover suppressor qC1 (Austin and Kimble, 1989) is a complex inversion (Fig. 1). The data used in this analysis were generated in the course of genome sequencing to identify causative lesions in lethal mutations balanced by qC1 (Li-Leger et al., 2021).

The primary inverted region in qC1 runs from LG III coordinates 1,286,123 through 13,737,951; these physical breakpoints correspond to the genetic limits of balancing behavior observed in recombination analysis of marker/qC1trans heterozygotes (dpy-1 to tra-1, summarized in Edgley et al., 1995). In addition to the primary inversion, the qC1chromosome is characterized by several regions exhibiting further rearrangement, possibly the result of the radiation dose used to generate the suppressor (7200 R gamma). The original balancer was known to carry lesions in glp-1 and mog-1 that were not present in the strain mutagenized to generate qC1 and not characterized molecularly; these lesions were both identified as additional inversion breakpoints inside the primary qC1 inversion. In particular, as can be seen in Fig. 1, glp-1(q339) consists of two breakpoints within the gene at chromosome coordinates 9,096,426 and 9,096,455, which in addition to breaking the gene deletes 28 base pairs from one of its coding exons. A similar event is seen in mog-1, with breakpoints at coordinates 9,955,633 and 9,955,971.

The schematic representation of qC1 chromosome III shown in Fig. 1B and1C is the simplest structure compatible with the breakpoints and breakpoint connections identified in the current study. However, we cannot exclude the possibility that additional breakpoints have been undetected, which would result in more complex chromosomal rearrangements.