Worm Breeder's Gazette 10(3): 144
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
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 possibilities? 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]