An X-autosome fusion chromosome of Caenorhabditis elegans

Abstract

The translocation mnT12(IV;X) is a fusion of holocentric chromosomes IV and X, the breakpoints occurring near the left end of IV and the right end of X. Animals homozygous for mnT12 are viable and fertile; they contain five pairs of chromosomes rather than the normal set of six pairs. The mnT12 chromosome is larger than all wild-type chromosomes and thus identifies linkage groups IV and X cytologically. Hermaphrodites heterozygous for mnT12 show high frequency meiotic nondisjunction both between mnT12 and the X chromosome, which results in a high incidence of male self progeny (27% compared to the wild-type incidence of 0.2%), and between mnT12 and chromosome IV, which results in a high incidence of self progeny essentially trisomic for chromosome IV (karyotype IV/mnT12/mnT12). The viability of chromosome IV trisomics has been confirmed by constructing animals trisomic for only normal copies of chromosome IV; these animals are morphologically wild type. Meiotic chromosome disjunction in mnT12 homozygotes appears to be normal, although the frequency of recombination between markers that are normally X-linked is significantly reduced. Males of genotype IV/mnT12/0 are fertile. They can be thought of as having a neo-X(mnT12) neo-Y(normal IV) karyotype since it is possible to maintain a male-hermaphrodite stock of C. elegans consisting of such males and hermaphrodites carrying two neo-X chromosomes and no neo-Y; the organism is thus converted from an XO:XX type of sex determination to an XY:XX system.

An Asymmetric Chromosome Pair Undergoes Synaptic Adjustment and Crossover Redistribution During Caenorhabditis elegans Meiosis: Implications for Sex Chromosome Evolution

“…C. elegans males do not carry out germline apoptosis and thus lack a mechanism to eliminate gametes prior to completion of meiosis (Gumienny et al 1999;Gartner et al 2000;JaramilloLambert and Engebrecht 2010). In addition, there is no evidence for increased meiotic nondisjunction in mnT12/1 males (Sigurdson et al 1986;Hillers and Villeneuve 2003). Thus, our observed crossover data likely reflect the actual distribution of crossovers in these animals.…”

“…In males heterozygous for the X;IV fusion chromosome mnT12 and normal IV, a region capable of homologous synapsis (the chromosome IV portion of mnT12) is contiguous with a chromosomal region with no homologous pairing partner (the X chromosome portion of mnT12) (Sigurdson et al 1986).…”

Aneuploidy: Cells Losing Their Balance

“…These worms were viable but were morphologically abnormal and exhibited slower growth as well as subfertility (Hodgkin et al 1979). Subsequent efforts led to the generation of worms trisomic for chromosome IV (Sigurdson et al 1986). These animals also are subfertile and exhibit morphological defects.…”

Molecular characterization of the her-1 gene suggests a direct role in cell signaling during Caenorhabditis elegans sex determination.

“…These experiments could not establish whether the rare 1.2-kb transcript was completely absent nor whether the presence of the 0.8-kb transcript could be accounted for by the low level of XO males that arise by X chromosome nondisjunction in N2 XX populations. To resolve these questions, we used the more sensitive technique of RNase protection to compare levels of the two transcripts in embryos of a him-8(e1489) strain (about 37% XO), N2 (-0.2% XO), and SP756 (-0.02% XO), a hermaphrodite strain in which X nondisjunction is suppressed by the X-to-IV translocation ranT12 (Sigurdson et al 1986). At optimal exposure levels for the him-8 RNA signal (Fig.…”