Use of fluorescent protein to analyse recombination at three loci in Neurospora crassa

Fluorescent Protein as a Tool for Investigating Meiotic Recombination in Neurospora

We have built a series of Neurospora crassa strains containing alleles of green fluorescent protein (GFP) to provide a visual phenotype for investigating meiotic recombination. These strains provide a convenient means of screening the Neurospora knockout library for genes involved in genetic recombination. They permit rapid analysis of recombination outcomes by allowing visualization of segregation patterns in a large number of octads from crosses heterozygous for GFP. Using this system the effect of a knockout on gene conversion and/or on crossing over between the fluorescent marker and the centromere can be measured.

Meiotic Recombination in Neurospora crassa Proceeds by Two Pathways with Extensive Holliday Junction Migration

Analysis of thousands of Δmsh-2 octads using our fluorescent recombination system indicates that, as in other filamentous fungi, symmetric heteroduplex is common in the his-3 region of Neurospora crassa. Symmetric heteroduplex arises from Holliday junction migration, and we suggest this mechanism explains the high frequency of His⁺ spores in heteroallelic crosses in which recombination is initiated cis to the his-3 allele further from the initiator, cog⁺. In contrast, when recombination is initiated cis to the his-3 allele closer to cog⁺, His⁺ spores are mainly a result of synthesis-dependent strand annealing, yielding asymmetric heteroduplex. Loss of Msh-2 function increases measures of allelic recombination in both his-3 and the fluorescent marker gene, indicating that mismatches in asymmetric heteroduplex, as in Saccharomyces cerevisiae, tend to be repaired in the direction of restoration. Furthermore, the presence of substantial numbers of conversion octads in crosses lacking Msh-2 function suggests that the disjunction pathway described in S. cerevisiae is also active in Neurospora, adding to evidence for a universal model for meiotic recombination.

Residual recombination in Neurospora crassa spo11 deletion homozygotes occurs during meiosis

Spo11 is considered responsible for initiation of meiotic recombination in higher organisms, but previous analysis using spo11 (RIP) mutants suggests that the his-3 region of Neurospora crassa experiences spo11-independent recombination. However, despite possessing several stop codons, it is conceivable that the mutants are not completely null. Also, since lack of spo11 interferes with chromosomal pairing and proper segregation at Meiosis I, spores can be partially diploid for a period after meiosis. Thus, it is possible that the recombination observed could be an abnormal event, occurring during the period of aneuploidy rather than during meiosis. To test the former hypothesis, we generated spo11 deletion homozygotes. Using crosses heteroallelic for his-3 mutations, we showed that His(+) progeny are generated in spo11 deletion homozygotes at a frequency at least as high as in wild type and, as in the spo11 (RIP) mutants, local crossing over is not reduced. To test the latter hypothesis, we utilised mutations in either end of a histone H1-GFP fusion gene, inserted between the recombination hotspot cog and his-3, in which GFP(+) spores arise as a result of recombination in a cross between the two GFP alleles. In a control cross homozygous for spo11 (+), the frequency at which GFP(+) spores arise is comparable to the frequency of His(+) spores and glowing nuclei first appear during prophase, prior to metaphase I, as expected for a product of meiotic recombination. Similarly in spo11 deletion homozygotes, GFP(+) spores arise at high frequency and glowing nuclei are first seen before metaphase, indicating that allelic recombination occurs during meiosis in the absence of spo11. We have therefore shown that spo11 is not essential for either his-3 allelic recombination or crossing over in the vicinity of his-3, and that spo11-independent allelic recombination is meiotic, indicating that there is a spo11-independent mechanism for initiation of recombination in Neurospora.