![]() crassa and both mapped to a region of 30 map units across the centromere of linkage group III. Both Sk-2 K and Sk-3 K were introgressed into the genetically better-characterized N. Only Sk-1 K occurs widespread in nature ( Turner and Perkins 1979). Several Spore killer types have been characterized in Neurospora: Sk-1 K from Neurospora sitophila and a Sk-2 K and Sk-3 K from N. In crosses homozygous for a killer allele ( Sk K × Sk K) each ascus contains eight viable black ascospores, as in normal sensitive crosses ( Sk S × Sk S), indicating that killing occurs only in crosses heterozygous for the killing factor ( Turner and Perkins 1979, 1991). All the viable spores carry the Sk K allele. Haploid Spore killer strains of Neurospora were originally identified because asci always contained four viable black and four small inviable unpigmented spores in crosses with standard wild-type strains. However, the best-studied example of meiotic drive in ascomycetes is Spore killer ( Sk) in Neurospora. Other fungi in which distorters have been found are Gibberella fujikuroi (= Fusarium moniliforme) and Cochliobolus heterostrophus (see Raju 1994, 1996 for a review). Turner and Perkins ( 1979, 1991) identified such abortion factors in Neurospora as Spore killers. The earliest analysis of two segregation distorters in fungi, then called ascospore abortion factors, is by Padieu and Bernet ( 1967) in the ascomycete Podospora. Thus distortion in fungi also affects the number of offspring produced and reduces the fecundity, which has important consequences for the population genetics of meiotic drive in fungi ( Nauta and Hoekstra 1993). The ascospores are the products of meiosis as well as the progeny. This is not the only distinguishing feature of drive systems in fungi. Any meiotic drive system in such fungi-provided the elimination of the nuclei containing the nondriving allele occurs in an early stage after the completion of meiosis, as it does in all known meiotic drive systems-will be observed in a cross between a driving and a sensitive strain as spore killing: the degeneration and early abortion of half the ascospores in a certain proportion of the asci. However, fungi in which the haploid nuclei resulting from meiosis are linearly arranged within an ascus provide unique opportunities to analyze abnormal segregation, for precisely the same reason that they have played such a big role in the classical experiments by Lindegren and others on fundamental aspects of linkage, meiotic recombination, and gene conversion (see Whitehouse 1973 Perkins 1992). For this reason it is understandable that an appreciable number of known cases of meiotic drive involve genes affecting the sex ratio. This is not easy to study because in animals and plants a driving genetic element requires a specific phenotype to be observable. Thus it threatens adaptive evolution and it is therefore of great interest to obtain information on the extent of meiotic drive in natural populations. ![]() Meiotic drive allows deleterious alleles to spread through populations if the frequency gain from their segregation advantage more than compensates the frequency loss due to elimination by natural selection. Furthermore, classes of insensitive target or suppressor alleles have accumulated to counter these selfish elements ( Lyttle 1991). Meiotic drive systems in these organisms showing –90% distortion are harder to detect. Likewise, distortion ratios for Drosophila and mice in nature may vary greatly. It is not known for most drive systems whether they involve two closely linked loci. Their ratio of distortion in these examples can exceed 90% and they are closely linked to the centromere. All distorters are associated with polymorphic chromosomal structures, such as inversions. In Drosophila and mouse, the meiotic drive systems minimally involve two closely linked loci, a distorter and its cis-acting target. Well-known examples of segregation distorters are the sex-ratio chromosomes ( SR) in Drosophila, a male sex chromosomal drive system, and the t-haplotype in mice and segregation distorter ( SD) in Drosophila, both male autosomal drive systems ( Lyttle 1991). SEGREGATION distorters are genetic elements that show meiotic drive, a phenomenon in which one member of a pair of heterozygous alleles is transmitted in excess of the expected Mendelian ratio of 50% ( Sandler and Novitski 1957 Lyttle 1991).
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