Worm Breeder's Gazette 11(5): 72

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The Effect of him Mutations on Recombination and Segregation

K.S. McKim and A.M. Rose

Figure 1

Meiotic mutations in the genes him-3(e1256)IV, him-6(e1423)IV and 
him-8(e1489)(IV) were originally isolated by Hodgkin et al.  (1979) 
because they caused a high level of X-chromosome nondisjunction.  Him-
6 and Him-3 hermaphrodites were found to have elevated levels of 
autosomal nondisjunction.  Him-8 hermaphrodites produced many male 
progeny due to random disjunction of their X-chromosomes.  Unlike the 
first two mutants, him-8 did not appear to affect autosomal 
disjunction.  I have tested the effects of him-3 and him-6 on 
recombination frequency in different regions of chromosomes I, III, V 
and X and tested the effects of all three mutants on the segregation 
patterns of free duplications.
Previous to this study, him-3 and him-6 were known to map on the 
right end of chromosome IV between dpy-13 and unc-31 (Figure).  In 
order to position them more accurately, him males were crossed to a 
series of deficiencies from the right end of chromosome IV.  him-3 was 
complemented by all the deficiencies tested.  him-6 failed to 
complement sDf2 and sDf60 but complemented mDf7.  This placed him-6 
close to unc-22 (Figure).  The frequency of males from him-6/sDf60 (
18/83 = 0.22) and him-6/sDf2 (49/264 = 0.19) hermaphrodites was 
slightly higher than in him-6/him-6 hermaphrodites (average of 13.7% 
in recombination experiments).  In addition, the number of progeny 
produced by him-6/sDfs hermaphrodites (279/7 = 40) was less than from 
him-6/him-6 hermaphrodites (average of 110 in recombination 
experiments).  Because the phenotype of e1423 was more severe when 
heterozygous to a deficiency, the e1423 mutation may be a hypomorph.
Recombination: Compared to the controls, him-3 mutants had a reduced 
recombination frequency on chromosome I.  The reductions were more 
severe in the bli-3  (25% of control) than in 
the dpy-5  (68% of control).  The severity of 
the reduction in the bli-3  of him-3 
hermaphrodites was confirmed when different markers (let-362 dpy-5) on 
the left end of chromosome I were used.  Similar results were found on 
chromosomes III where him-3 mutants reduced recombination in the dpy-
17 unc-64 interval (65% of control) and to a lesser degree in the unc-
45  (80% of control).  On chromosome V, 
recombination in the unc-60  was reduced to 
50% of the control frequency.
On the X-chromosome, him-3 had little effect on recombination 
frequencies.  The recombination frequency in the unc-1 dpy-7 interval 
was 82% of controls and in the dpy-7 unc-3 interval the recombination 
frequency 97% of controls.  Considering the high level of X-chromosome 
nondisjunction in Him-3 hermaphrodites, approximately 15% of the 
progeny from him-3 hermaphrodites were males, the small reduction on X 
chromosome recombination was surprising.  In addition, the lack of an 
effect of him-3 on X-chromosome recombination differed from the 
observations made on the autosomes.
him-6 also caused reductions in recombination frequency.  In the bli-
3 unc-11 region, recombination frequency was reduced in him-6 (75% of 
control) but not to the degree of reduction in him-3 mutants.  There 
appeared to be no effect of him-6 in the dpy-5 unc-54 region.  In 
contrast, recombination was reduced over the entire lengths of 
chromosome III and chromosome V.  Recombination was also generally 
reduced on the X-chromosome.  Unlike the situation with him-3, the X-
chromosome nondisjunction phenotype of him-6 correlated with the 
reduction in recombination frequency.
Autosomal nondisjunction.  The level of chromosome I nondisjunction 
in him-3 and him-6 hermaphrodites was assayed by crossing hT2/+ males 
to dpy-5; unc-64; unc-36; 
odites.  If a disomy I gamete was produced by 
the Him strain, then there was a 25% chance it would be fertilized by 
a I(R)III(R)hT2; IIIN sperm to produce a dpy-5 worm (the I(R)III(R)hT2;
III(N) chromosome of hT2(I;III) does not carry dpy-5(+).  In the 
control, hT2/+ males were crossed to dpy-5; 
odites.  No Dpy or Unc progeny were observed 
in 1882 progeny.  Thus, the normal chromosome I nondisjunction 
frequency was less than 0.002.
From the him-3 cross 14 dpy-5 hermaphrodites and 4 Dpy-5 males were 
recovered in 114 wild-type hermaphrodites and 106 wild-type males.  
The excess of Dpy-5 hermaphrodites may have resulted from viability 
differences between hermaphrodites and males or from a segregation 
pattern such that disomic I gametes were often disomic X as well.  The 
frequency of chromosome I nondisjunction in the Him-3 oocytes was (
18/238) 0.076.  In the him-6 cross, seven Dpy-5 hermaphrodites and 
five Dpy-5 males were recovered in 471 wild-type hermaphrodites and 
451 wild-type males.  In this case there was no excess of Dpy-5 
hermaphrodites.  The frequency of chromosome I nondisjunction in the 
Him-6 oocytes was (12/934) 0.013.
Effects on duplication segregation: We tested the effects of him-3, 
egregation of chromosomal 
duplications.  In scoring the self-fertilization progeny of hDpx; 
odites, the him mutants did not increase the 
frequency of chromosome loss (mitotic and meiotic).
The effects of the him mutants on the segregation of duplications 
from the X chromosome was tested by crossing dpy-5; 
him-y males to dpy-5; 
odites.  In normal males, hDp12 segregated 
from the X-chromosome by a non-homologous process (see Herman et al.  
1979).  him-3 disrupted the nonhomologous segregation of hDp12 
resulting in equal numbers of male and hermaphrodite wild-types and 
Dpy progeny.  In contrast, him-6 and him-8 did not effect non-
homologous segregation of hDp12 males.  hDp31 carries part of the left 
end of the X-chromosome joined to chromosome I sequences and 
homologously pairs and recombines with the X chromosome.  hDp31 
continued to segregate from the X-chromosome in both him-3 and him-8 
males, although the efficiency of segregation was reduced compared to 
controls.  The segregation of hDp31 in these strains might be by the 
nonhomologous process.
[See Figure 1]

Figure 1