Achiasmate Meiosis
Achiasmate Meiosis refers to
can occur.The traditional line of thinking was that without at least one chiasma between homologs, they could not be properly segregated during
Chromosomal segregation strategies
Chiasmata play a crucial role in correctly segregating the chromosomes during meiosis I to maintain correct ploidy; when chiasmata fail to form, it typically results in aneuploidy and nonviable gametes.[2] However, some species have been found to employ alternative methods to segregate chromosomes.[5] They all involve linking the homologs together with some structure. These structures provide the same needed tension that chiasmata usually provide.
Synaptonemal complex and centromere interaction
One segregation strategy is to create a
Heterochromatin
Heterochromatin is a tightly grouped type of DNA. Threads of heterochromatin have been observed in Drosophila melanogaster, connecting achiasmate homologs and allowing them to move pull back and forth by spindles as a connected duo.[6][7]
Known achiasmatic species
Saccharomycodes ludwigii
While multiple species of budding yeast have been found to have residual SC proteins that connect the centromeres together when needed, nearly all of said species are chiasmatic and have been simply used as convenient model organisms.
Drosophila melanogaster
In Drosophila melanogaster, both oocytes and spermatocytes display achiasmy. In oocytes, neither the 4th nor the sex-determining chromosomes form chiasmata; in spermatocytes, no chiasmata form on any of the chromosomes.[7][9] Heterochromatin threads have been observed in D. melanogaster oocytes.[6] Unusually, D. melanogaster lack SCs all together, so SC proteins likely do not play a role in this species' segregation strategy.[9]
Amazon Molly
Amazon Mollies (Poecilia formosa) reproduce without recombination via gynogenesis. They mate with males of other species and the sperm triggers the development of their eggs, but the Amazon Mollies create diploid eggs that have copies of only their own genes.[4] There is no crossing over during their meiosis, indicating that they have achiasmate meiosis. It is theorized that this failure during the meiotic cycle is what creates the diploid eggs and that likely sister chromatids are separated during meiosis instead of the homologs in this species.[4] If sister chromatids are being separated instead of homologs, than proper segregation of homologs has failed in this species.
Insects
True bugs (order Heteroptera) are partially of achiasmate species and partially of chiasmate species in reference to spermatogenesis. The infraorder Cimicomorpha, specifically its families Anthocoridae, Microphysidae, Cimicidae, Miridae, and Nabidae are achiasmate. Additionally, achiasmy has been reported in the infraorder Leptopodomorpha and in the family Micronectidae on the infraorder Nepomorpha.[10][11] A deeper understanding of how meiosis proceeds in these achiasmate species is still under investigation.
Evolution
It is thought that achiasmatic meiosis is polyphyletic, as there is no distinct pattern to its occurrence, nor to the methods through which it occurs. It appears to instead be multiple instances of secondary loss of meiotic recombination that resulted in either the evolution of new segregation processes, or a shift to an existing backup system for segregation.[5] Current evidence suggests the latter, that there are existing mechanisms to segregate homologs without chiasmata, as these mechanisms (heterochromatin and centromere-centromere interaction) have been observed in chiasmate species.[12][2]