Gene conversion: a non-Mendelian process integral to meiotic recombination

Abstract

Meiosis is undoubtedly the mechanism that underpins Mendelian genetics. Meiosis is a specialised, reductional cell division which generates haploid gametes (reproductive cells) carrying a single chromosome complement from diploid progenitor cells harbouring two chromosome sets. Through this process, the hereditary material is shuffled and distributed into haploid gametes such that upon fertilisation, when two haploid gametes fuse, diploidy is restored in the zygote. During meiosis the transient physical connection of two homologous chromosomes (one originally inherited from each parent) each consisting of two sister chromatids and their subsequent segregation into four meiotic products (gametes), is what enables genetic marker assortment forming the core of Mendelian laws. The initiating events of meiotic recombination are DNA double-strand breaks (DSBs) which need to be repaired in a certain way to enable the homologous chromosomes to find each other. This is achieved by DSB ends searching for homologous repair templates and invading them. Ultimately, the repair of meiotic DSBs by homologous recombination physically connects homologous chromosomes through crossovers. These physical connections provided by crossovers enable faithful chromosome segregation. That being said, the DSB repair mechanism integral to meiotic recombination also produces genetic transmission distortions which manifest as postmeiotic segregation events and gene conversions. These processes are non-reciprocal genetic exchanges and thus non-Mendelian.

Fig. 1. Schematic showing the Mendelian 4:4 segregation pattern, as well as the non-Mendelian 5:3/3:5 (PMS) and 6:2/2:6 (gene conversion) segregation patterns of a single heterozygous site.

Recombination performed nearby such a heterozygous site will generate heteroduplex DNA (hDNA) containing mismatches which can lead to non-Mendelian segregation events.

Fig. 2. Models of intersister and non-crossover DSB repair pathways during meiosis.

A homologous chromosome pair is represented by blue and red sister chromatids. For clarity only the chromatids involved in recombination are shown, except for the initial step of the pathways and the final step of canonical Synthesis-Dependent Strand Annealing (SDSA). Please, note that the small gap representing the DSB does not indicate loss of genetic material as with Spo11 double cutting (see Fig. 3). Early steps of recombination are shown: DSB formation by Spo11, DNA strand resection to expose 3’ single-stranded tails which then invade a homologous template to form Displacement loops (D-loops). D-loops can be dissociated (by the action of DNA helicases; reviewed in Lorenz 2017) before or after DNA synthesis has started; antirecombination driven by MutSα, MutSβ, and MutLα also plays a key role here (see main text). Canonical SDSA is thought to produce non-crossover gene conversion events. The position of the initiating DSB site is indicated by a green vertical line. Dependent on the actions of mismatch repair a given heterozygous site within hDNA can be left unrepaired (PMS, 5:3 segregation), converted (gene conversion, 6:2 segregation), or restored (Mendelian 4:4 segregation) (see main text and Fig. 1 for details). Multiple invasion/dissociation cycles can result in complex conversion events when template switches between sister chromatids and homologues occur. The simultaneous or consecutive invasion of both ends of the DSB into the same or different chromatids of the homologue can result in double SDSA, were conversion tracts left and right of the original DSB site can be detected.

Fig. 3. Model of direct formation of gene conversion (stretch of 6:2 segregation) and PMS (stretches of 5:3 segregation) at a gap created by two Spo11 DSBs in close proximity to each other.

For the sake of simplicity only a single sister chromatid (one in blue, one in red) per homologous chromosome is shown, except for the last step to illustrate the segregation patterns of the recombination outcome. The positions of the initiating DSB sites are indicated by green vertical lines.

Fig. 4. Models of crossover DSB repair pathways during meiosis.

A homologous chromosome pair is represented by blue and red sister chromatids. For clarity only the chromatids involved in recombination are shown after the initial step of the pathways. Please, note that the small gap representing the DSB does not indicate loss of genetic material as with Spo11 double cutting (see Fig. 3). The early steps of recombination are shown as before (see Fig. 2). These include: DSB formation by Spo11, 5’ to 3’ resection of DSB ends on the broken DNA duplex (blue) to expose 3’ single-stranded tails, and strand invasion of the intact DNA duplex (red) to form a Displacement loop (D-loop). For crossover repair pathways, D-loop formation is then followed by DNA synthesis, helicase-mediated branch migration (grey arrows), capture of the second end of the DSB, further processing to produce double Holliday junctions, and biased resolution of these double Holliday junctions into crossovers. The position of the initiating DSB site is indicated by a green vertical line. Left: DNA synthesis of the invading end only, followed by limited branch migration and further processing, results in a crossover with hDNA in each chromatid and on opposite sides of the DSB. If branch migration is more extensive, moving the single-end invasion intermediate away from the initiating DSB site, resolution results in a crossover with hDNA on one chromatid and on the left side of the DSB. Centre: DNA synthesis of the captured end only, followed by branch migration and further processing, results in a crossover with hDNA on one chromatid and on the right side of the DSB. Right: DNA synthesis of both the invading end and the captured end, followed by branch migration and further processing, results in a crossover with hDNA on one chromatid and on both sides of the DSB. Note that branch migration is depicted as unidirectional (to the right) for simplicity but can occur in the opposite direction as well. Branch migration is also possible after annealing.