In a heterozygous Spore killer cross, the four Sk-2 (black/melanized) progeny develop normally, whereas the four non– Sk-2 (white and small) progeny abort.
In a typical ( sensitive × sensitive) cross, each mature ascus contains eight spindle-shaped (American football-like) ascospores. Phenotypes of Sk S (sensitive), Sk-2, and r(Sk-2) crosses. This analysis placed r(Sk-2) to the right of AFLP1 and to the left of ASP1, eliminating all genes except 09151, 09152, 09153, and 09154.įig. For example, progeny #34 has the LA pattern for AFLP1 and the OR pattern for the other three markers ( Fig. Amplified fragment length polymorphism (AFLP) and amplified sequence polymorphism (ASP) markers were then used to determine the approximate crossover point in two progeny (#34 and #119). The genotypes of the recombinant progeny indicate that r(Sk-2) LA should lie between the two hph markers ( Tables S1 and S2). These strains were then crossed to a cum r(Sk-2) acr-7 strain (LA P8-11) to create two mapping populations. Hygromycin resistance genes were placed in a sensitive (OR) background to generate strains with markers between genes 0910 ( hph 49/50 P17-13) or between genes 0916 ( hph 55/56 P17-12). ( B) The r(Sk-2) locus was bracketed by genetic crossovers. Sk-2 and Sk-3 are located in a 30-map unit region within which recombination is blocked in heterozygous ( Sk × WT) crosses. ( A) The Spore killer region on chromosome III. Genetic locations of Sk-2, Sk-3, and r(Sk-2). The Sk-2 and Sk-3 killers do not encode resistance to each other, and all eight progeny are inviable when the two mate.įig. These observations suggest that the killer produces a resistance factor that shelters all nuclei residing in the same spore. crassa mutant and in the four-spored (heterokaryotic) Neurospora tetrasperma, they can escape the elimination process ( 7, 8). When sensitive nuclei are enclosed in the same spore with the killer ones, as seen in a giant-spored N. Homozygous killer × killer and sensitive × sensitive crosses are normal and yield 8B:0W asci (spore sacs). Manifestation of killing does not occur until late ascospore development: Degeneration of sensitive spores takes place after the normal progression of meiosis, spore delimitation, and the second postmeiotic mitosis ( 6). In a Spore killer ( Sk) × WT ( Sk-sensitive or Sk S) cross, regardless of which acts as the female, the four Sk-containing spores are black (B) and viable, whereas the four Sk-sensitive spores are white (W) and inviable. intermedia, the two spore-killing factors have been introgressed into Neurospora crassa for extensive genetic studies. Spore killer-2 ( Sk-2) and Spore killer-3 ( Sk-3), which behave similarly, are the most studied distortion elements in Neurospora ( 5). Rather, a resistant version of RSK likely neutralizes the killer element and prevents it from interfering with ascospore development. The WT RSK protein is dispensable for ascospore production and is not a target of the spore-killing mechanism.
Contrary to long-held belief, the killer targets not only late but also early ascospore development. Sk-2, Sk-3, and sensitive rsk alleles differ from each other by their unique indel patterns. In each killer system, rsk sequences from an Sk strain and a resistant isolate are highly similar, suggesting that they share the same origin. Sk-2, Sk-3, and naturally resistant isolates all use rsk for resistance. rsk seems to be a fungal-specific gene, and its deletion in a killer strain leads to self-killing. Here we report the identification and characterization of the Sk resistance gene, rsk ( resistant to Spore killer). Sk-2 and Sk-3 are the most studied meiotic drive elements in Neurospora, and they each theoretically contain two essential components: a killer element and a resistance gene. In a cross of Spore killer ( Sk) × WT ( Sk-sensitive), the ascospores containing the Spore killer allele survive, whereas the ones with the sensitive allele degenerate. In Neurospora, meiotic drive can be observed in fungal spore killing. In some cases, progeny or gametes carrying a meiotic drive element can survive preferentially because it causes the death or malfunctioning of those that do not carry it. Meiotic drive is a non-Mendelian inheritance phenomenon in which certain selfish genetic elements skew sexual transmission in their own favor.
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