Spontaneous mutations were allowed to accumulate in a second chromosome that was transmitted only through heterozygous males for 40 generations. At 10-generation intervals the chromosomes were assayed for homozygous effects of the accumulated mutants. From the regression of homozygous viability on the number of generations of mutant accumulation and from the increase in genetic variance between replicate chromosomes it is possible to estimate the mutation rate and average effect of the individual mutants. Lethal mutations arose at a rate of 0.0060 per chromosome per generation. The mutants having small effects on viability are estimated to arise with a frequency at least 10 times as high as lethals, more likely 20 times as high, and possibly many more times as high if there is a large class of very nearly neutral mutations.—The dominance of such mutants was measured for chromosomes extracted from a natural population. This was determined from the regression of heterozygous viability on that of the sum of the two constituent homozygotes. The average dominance for minor viability genes in an equilibrium population was estimated to be 0.21. This is lower than the value for new mutants, as expected since those with the greatest heterozygous effect are most quickly eliminated from the population. That these mutants have a disproportionately large heterozygous effect on total fitness (as well as on the viability component thereof) is shown by the low ratio of the genetic load in equilibrium homozygotes to that of new mutant homozygotes.

Loss of genomic imprinting is involved in a number of developmental abnormalities and cancers. ZAC is an imprinted gene expressed from the paternal allele of chromosome 6q24 within a region known to harbor a tumor suppressor gene for several types of neoplasia. p57KIP2 (CDKN1C) is a maternally expressed gene located on chromosome 11p15.5 which encodes a cyclin-dependent kinase inhibitor that may also act as a tumor suppressor gene. Mutations in ZAC and p57KIP2 have been implicated in transient neonatal diabetes mellitus (TNDB) and Beckwith–Wiedemann syndrome, respectively. Patients with these diseases share many characteristics. Here we show that mouse Zac1 and p57Kip2 have a strikingly similar expression pattern. ZAC, a sequence-specific DNA-binding protein, binds within the CpG island of LIT1 (KCNQ1OT1), a paternally expressed, anti-sense RNA thought to negatively regulate p57KIP2 in cis. ZAC induces LIT1 transcription in a methylation-dependent manner. Our data suggest that ZAC may regulate p57KIP2 through LIT1, forming part of a novel signaling pathway regulating cell growth. Mutations in ZAC may, therefore, contribute to Beckwith–Wiedemann syndrome. Furthermore, we find changes in DNA methylation at the LIT1 putative imprinting control region in two patients with TNDB.

In a marked-inversion-balanced lethal system, mutations were accumulated at a minimum pressure of natural selection on 2000 second chromosomes of Drosophila melanogaster that originated from 4 stem chromosomes. Five enzyme loci were tested: a-glycerol-3-phosphate dehydrogenase (EC 1.1.1.8), malate dehydrogenase (Mdh, EC 1.1.1.37), alcohol dehydrogenase (EC 1.1.1.1), hexokinase-C (Hex-C tentative name), and a-amylase (Amy, EC 3.2.1.1). Three band-morph mutants, one at the Mdh locus, one at the Hex-C locus, and one at the Amy locus, were detected out of 1,658,308 allele replications. In addition, 17 null mutants were found. Accepting that the number of structural genes is the same as that of bands in the salivary gland chromosomes, the total mutation rate per generation for all the structural genes in the second chromosomes is estimated to be 0.008-0.040, which is much smaller than that estimated for viability polygenes (0.12-0.17). Thus, it is speculated that most viability and other fitness polygenes are located in controlling regions outside the structural genes. Large numbers of protein polymorphisms have been reported in many species since Lewontin and Hubby (1) published data for Drosophila pseudoobscura. Although several mechanisms for the maintenance of these polymorphisms have been proposed, a decisive conclusion on this matter has not been reached. One of the reasons for this is that a reliable estimate for the spontaneous mutation rate has not been obtained.In Drosophila melanogaster, there are two reports (2, 3) of direct estimates of band-morph mutation rates for enzyme loci determined by electrophoresis, but the results are not very reliable because no system of checking for contamination was used.Extensive studies were made of mutation rates of viability polygenes in D. melanogaster (4, 5). The results indicated that the viability polygenic mutation rate is approximately 20 times higher than the recessive lethal mutation rate on a chromosome basis. Unfortunately, these estimates were obtained by indirect statistical methods and the nature of these mutations was not known. From developments in molecular genetics and other evidence, it may be possible to say something about the location and basis of polygenic mutations.The purposes of the present work are twofold: (i) to estimate the rate of occurrence of band-morph mutations, from which the mutation rate per base-pair site may be estimated; (ii) to focus this and other evidence onto the nature of polygenic mutations. MATERIALS AND METHODSFour stem chromosomes were used: two SMI (Cy) chromosomes and two unrelated lethal-carrying chromosomes, t(AW) and t(JH), which were derived in 1967 from a cage population (W-1) (6). A single male, which was a heterozygote for SMJ (Cy) and t(AW), was mated to a single C-160 [SMI (Cy)/In(2LR)-btvl, which is abbreviated Cy/Pm] female. To establish the chromosome lines from the progenies, Cy/e males and females were collected and many single-pair matings were made between them. In the progenies, generation one...

We present the complete nucleotide and deduced amino acid sequence for the gene encoding Drosophila sn-glycerol-3-phosphate dehydrogenase. A transcription unit of 5kb was identified which is composed of eight protein encoding exons. Three classes of transcripts were shown to differ only in the 3'-end and to code for three protein isoforms each with a different C-terminal amino acid sequence. Each transcript is shown to arise through the differential expression of three isotype-specific exons at the 3'-end of the gene by a developmentally regulated process of 3'-end formation and alternate splicing pathways of the pre-mRNA. In contrast, the 5'-end of the gene is simple in structure and each mRNA is transcribed from the same promoter sequence. A comparison of the organization of the Drosophila and murine genes and the primary amino acid sequence between a total of four species indicates that the GPDH gene-enzyme system is highly conserved and is evolving slowly.