![]() Furthermore, meiotic drivers can cause infertility. For example, meiotic drivers can only be observed in heterozygotes, where a given drive allele has a competitor. These factors are important to consider as there can be differences between gametogenesis in homozygotes versus heterozygotes. In addition, heterozygotes generated by crossing natural isolates are not always highly fertile. In the wild, extreme inbreeding is generally not a winning evolutionary strategy and gametogenesis often occurs in heterozygotes. This choice of experimental systems has facilitated innumerable discoveries, but the choice has also given a biased view of how sexual reproduction happens. homozygous) highly inter-fertile organisms for their experiments. Perhaps the biggest contributor inhibiting the discovery of meiotic drive loci is that geneticists often follow Mendel’s example and choose pure-breeding (i.e. In this way, meiotic drivers can contribute to hybrid infertility and speciation. If these two populations were to merge ( v), the phenotypes of cryptic drivers could emerge in hybrid individuals. v) Over time, populations (represented in ii and iv) evolve distinct landscapes of meiotic drivers and suppressors. iv) As drivers are generally deleterious for fitness, unlinked suppressors ( c) are likely to evolve. The original drive locus could accumulate mutations and be lost in the population. iii) Another possible path is that the driver ( A) could duplicate, creating a drive locus ( b) that is free to diverge. ii) If a drive allele becomes fixed in a population, drive will not be observed because all individuals will be homozygous. I) A meiotic drive locus ( A) has the ability to generate drive when heterozygous. Hypothetical evolution of meiotic drivers We omitted discussion of meiotic drivers that either act during the meiotic divisions or have an unknown mode of altering allele transmission frequencies. ![]() Here, we review these exciting discoveries and examine their implications. Several genes used in killer meiotic drive systems have been identified. Some killers even destroy both male and female gametes that do not inherit them. However, there are also killer meiotic drivers that act on gametes produced by female (asymmetric) meiosis in which only one meiotic product becomes a gamete. Killers often act on the products of male (symmetric) meiosis in which all meiotic products are viable. In other words, they can spread through a population by actively destroying competitors. Instead, killers gain an advantage by sabotaging otherwise viable meiotic products that do not inherit the selfish allele. These ultra-selfish alleles do not achieve a transmission bias by increasing the absolute number of meiotic products carrying them. These drivers are sometimes called ‘ultra-selfish’ genes because of their exceptional methods. The second class of drivers are the killer meiotic drivers that constitute the focus of this review. For example, loci called ‘knobs’ in maize can promote their preferential segregation during meiosis to the position that will become the egg. Class one drivers, sometimes called ‘ true meiotic drivers’ (see glossary), act during the meiotic chromosome divisions. There are two general classes of meiotic drivers. Meiotic drive is thus a powerful evolutionary force that has likely shaped the structure of eukaryotic chromosomes and the genes that act during meiosis and gametogenesis. The transmission advantage enjoyed by these cheaters can allow them to spread in a population, even if they are deleterious to fitness, which they often are. These selfish loci can bias allele transmission in their favor so that they are found in more than half of the functional meiotic products generated by a heterozygote. Meiosis and gametogenesis are, however, vulnerable to exploitation by meiotic drivers. Unbiased segregation of homologous chromosomes during meiosis is now known to underlie Mendelian (equal) allele transmission. This law stipulates that each of the two alleles carried by a heterozygote will be transmitted equally into meiotic products (e.g. ![]() Two of Mendel’s ‘Laws’ (Dominance and Independent Assortment) have long since been downgraded and are now better described as ‘context dependent guidelines.’ His Law of Segregation, however, has endured. The visionary monk Gregor Mendel was the first to describe the rules governing the transmission of alleles into offspring.
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