Turner and Perkins (1979) first showed that ascospore death in certain crosses of N. sitophila and N. intermedia resulted from the action of an ascospore abortion factor they named Spore killer (Sk). Three different Spore killers are known in Neurospora: Sk-1 in N. sitophila, and Sk-2 and Sk-3 in N. intermedia. Sk-2 and Sk-3 have since been introgressed into N. crassa and N. tetrasperma for genetic analysis (see Turner and Perkins 1991; Raju ad Perkins 1991). As with other segregation distorters in animals and plants, fungal Spore killers have been found only among strains from nature (Turner and Perkins 1979; Turner 2001).
When a cross is heterozygous for a killer element (SkK x SkS), each ascus produces four large, black, viable ascospores and four small, hyaline, inviable ascospores (Figs. 1-3, 12, 13, 16-19). The result is gross distortion of allele ratios for genes linked to Sk on linkage group III. There is little or no ascospore death in homozygous killer x killer (Figs. 4, 5, 9, 10) or sensitive x sensitive (Fig. 11) crosses. In crosses heterozygous for a Spore killer, crossing over is blocked in a 30 map-unit region that spans the centromere of LG III.
In SkK x SkS, where killing occurs, chromosome pairing, meiosis and the postmeiotic mitoses are completely normal and eight apparently normal ascospores are cut out in each ascus. The four ascospores that carry SkS are arrested shortly after the first mitosis in the young ascospores, and they gradually shrink and abort (Figs. 16-19). A sensitive nucleus that would otherwise die is rescued, unchanged, if a killer nucleus is also included in the same ascospore (Raju 1979). This has been shown for a developmental giant-ascospore mutant of N. crassa (Fig. 20), and for the naturally heterokaryotic ascospores of N. tetrasperma and Podospora anserina (Raju and Newmeyer 1977; Raju and Perkins 1991; Raju 1994, 2002). Spore killer-2 has been used to map centromere distances of mutant genes in N. crassa from unordered tetrads that have been ejected from asci (Figs. 21, 22; Perkins et al. 1986).
Photographs of Spore killer expression in various crosses are shown here in Figs.1-22.
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Fig 1. N. sit, Sk1 x WT |
Fig 2. N. sit, Sk1 x WT |
Fig 3. N. sit, Sk1 x WT |
Fig 4. N. sit, Sk1 x Sk1 |
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Fig 5. N. sit, Sk1 x Sk1 |
Fig 6. N. cra, Sk2 x WT |
Fig 7. N. cra, Sk2 x WT |
Fig 8. N. cra, Sk2 x WT |
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Fig 9. N. cra, Sk2 x Sk2 |
Fig 10. N. cra, Sk2 x Sk2 |
Fig 11. N. cra, WT x WT |
Fig 12. N. int, Sk3 x WT |
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You may use any of these photos for educational purposes (not for profit). We ask that you use a credit line citing the original reference and the photographer. High-resolution versions of these photos are available upon request from N.B. Raju’s photo collection, see Contacts. If previously published photos are to be used in publication, permission will, of course, be required from the publisher (see the captions of enlarged photos).
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