Worm Breeder's Gazette 10(3): 144

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Free Duplications and Maternal Rescue

Anne M. Villeneuve and Barbara J. Meyer

Figure 1

Many free Dps are transmitted to only a relatively small fraction of 
the offspring of Dp-bearing hermaphrodites.  Estimated rates of 
mitotic loss of free Dps in somatic tissue (calculated for Dps that 
have been used to generate somatic mosaics) are too low to account for 
the large number of nullo-Dp gametes produced.  The large proportion 
of gametes lacking the Dp must reflect a higher rate of mitotic loss 
in the germline than in the soma, a high rate of meiotic loss, or some 
combination of the two.  How can we distinguish among these 
Using strains that were constructed for a mosaic analysis of sdc-1 (
which we will report on at a later date), we have made some 
observations that address this issue.  sdc-1 mutations result in an 
incompletely penetrant transformation of XX animals toward the male 
fate.  In the particular strain used, the penetrance of the Tra 
phenotype is 58%.  This Tra phenotype exhibits complete maternal 
rescue, so that all sdc-1 homozygous progeny of heterozygous sdc-1/+ 
mothers are wild-type in sexual phenotype.  Likewise, a hermaphrodite 
carrying a wild-type copy of the sdc-1 locus on an attached Dp (mnDp1) 
provides maternal rescue to all of her non-Dp-bearing (sdc-1(-)) 
progeny.  A hermaphrodite carrying a wild-type copy of sdc-1 on a free 
Dp (mnDp12), however, fails to maternally rescue many (perhaps most) 
of her non-Dp-bearing sdc-1 progeny.  The penetrance of the Tra 
phenotype among these nullo-Dp progeny is 31% (see table).  (Both Dp-
bearing and nullo-Dp Tra animals are produced at relatively constant 
rates throughout the course of progeny production.)
mnDp12 is transmitted to only 24% of the self-progeny of Dp-bearing 
hermaphrodites (Herman, Genetics 108: 165-157).  The lack of maternal 
rescue of sdc-1 by mnDp12 implies that this low rate of transmission 
of mnDp12 cannot be accounted for by meiotic loss alone, since the 
attached Dp mnDp1, which gives rise to nullo-Dp progeny via meiotic 
segregation of the Dp, does provide maternal rescue of sdc-1.  The 
simplest explanation for the lack of maternal rescue with mnDp12 is 
that a large fraction (we estimate 40-50% or more based on the 
penetrance of the sdc-1 e) of germ cell 
nuclei lack the Dp due to mitotic loss; that is, the Dp was never 
present in a large fraction of the oocytes.  Since the 1000 germ cell 
nuclei present at the end of larval development derive from the single 
germline precursor P4 via exponential expansion, we can calculate the 
rate of mitotic loss of mnDp12 in the germline to be about 4-5% per 
cell division, a rate approximately 8-10 times higher than our 
experimentally determined rate for somatic loss of mnDp12.The absence 
of maternal rescue of sdc-1 by the free Dp mnDp12 has several other 
implications.  First, all mnDp12 hermaphrodites are apparently 
germline mosaics, since all give rise to non-rescued sdc-1 progeny.  
We can infer that the maternally-rescuing sdc-1(+) activity is 
supplied by the germline itself rather than by some other tissue (such 
as intestine, which provides yolk proteins).  A similar strategy using 
other free Dps might be applicable for determining the source of 
rescuing activity for other maternal-effect genes.
Second, the sdc-1 progeny arising from germ cell nuclei that have 
suffered a mitotic loss of the duplication are not maternally rescued 
even though those nuclei reside in a common syncytial cytoplasm with 
nuclei that retain the Dp.  One possible explanation is that the 
effects of sdc-1(+) expression may be local in nature--that is, the 
wild-type gene must be present in a given nucleus (or perhaps in a 
nearby nucleus) in order to provide rescuing activity to the zygote 
ultimately derived from that nucleus.  Indeed there is precedent for 
restricted spatial localization of RNA and protein products in 
syncytial tissues, both in the Drosophila syncytial blastoderm embryo 
and in vertebrate skeletal muscle.  Alternatively, a 2- to 5-fold 
dilution of the sdc-1(+) product in the syncytial cytoplasm resulting 
from lack of the Dp in 50-80% of germ cell nuclei might reduce a 
freely diffusible maternal rescuing activity below a hypothetical 
threshold level required for rescue.
[See Figure 1]

Figure 1