Achiasmate Meiosis

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Achiasmate Meiosis refers to meiosis without chiasmata, which are structures that are necessary for recombination to occur and that usually aid in the segregation of non-sister homologs. [1] The pachytene stage of prophase I typically results in the formation of chiasmata between homologous non-sister chromatids in the tetrad chromosomes that form. [1] The formation of a chiasma is also referred to as crossing over. When two homologous chromatids cross over, they form a chiasma at the point of their intersection. However, it has been found that there are cases where one or more pairs of homologous chromosomes do not form chiasmata during pachynema. [2] [3] [4] Without a chiasma, no recombination between homologs can occur.

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The traditional line of thinking was that without at least one chiasma between homologs, they could not be properly segregated during metaphase because there would be no tension between the homologs for the microtubules to pull against. [5] This tension between the homologs is typically what allows the chromosomes to align along an axis of the cell (the metaphase plate) and to then properly segregate to opposite sides of the cell. Despite this, achiasmate homologs are still found to line up with the chiasmate chromosomes at the metaphase plate. [6]

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 centromere-centromere interaction between achiasmate homologous chromosomes. Residual proteins from the synaptonemal complex (SC) ‘stick’ between the homologs' centromeres after diplotene, when the SC typically dissociates, allowing the homologs to achieve biorientation and attach correctly to the microtubules during anaphase I. [2] This has been observed in budding yeast, Drosophila melanogaster, and mouse spermatocytes. [2]

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. [2] [3] However, Saccharomycodes ludwigii also displays centromere-centromere interactions with SC proteins and is also almost entirely achiasmatic. It employs the breeding strategy of automixis (commonly used by many budding yeasts) in addition with a nearly complete lack of genetic mixing via crossovers to gain the genetic/evolutionary advantages of cloning (asexual reproduction) while maintaining the heterozygosity typically afforded by sexual reproduction. [8] S. ludwigii also creates strong connections between the tetrads produced by meiosis to promote the breeding (automixis) within the tetrad. This breeding strategy may have evolved “through mutual selection between suppression of meiotic recombination and frequent intratetrad mating", which would have helped the trait spread to fixation. [8]

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]

Related Research Articles

<span class="mw-page-title-main">Meiosis</span> Cell division producing haploid gametes

Meiosis is a special type of cell division of germ cells and apicomplexans in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately result in four cells with only one copy of each chromosome (haploid). Additionally, prior to the division, genetic material from the paternal and maternal copies of each chromosome is crossed over, creating new combinations of code on each chromosome. Later on, during fertilisation, the haploid cells produced by meiosis from a male and a female will fuse to create a cell with two copies of each chromosome again, the zygote.

<span class="mw-page-title-main">Chromosomal crossover</span> Cellular process

Chromosomal crossover, or crossing over, is the exchange of genetic material during sexual reproduction between two homologous chromosomes' non-sister chromatids that results in recombinant chromosomes. It is one of the final phases of genetic recombination, which occurs in the pachytene stage of prophase I of meiosis during a process called synapsis. Synapsis begins before the synaptonemal complex develops and is not completed until near the end of prophase I. Crossover usually occurs when matching regions on matching chromosomes break and then reconnect to the other chromosome.

<span class="mw-page-title-main">Prophase</span> First phase of cell division in both mitosis and meiosis

Prophase is the first stage of cell division in both mitosis and meiosis. Beginning after interphase, DNA has already been replicated when the cell enters prophase. The main occurrences in prophase are the condensation of the chromatin reticulum and the disappearance of the nucleolus.

<span class="mw-page-title-main">Homologous chromosome</span> Chromosomes that pair in fertilization

A couple of homologous chromosomes, or homologs, are a set of one maternal and one paternal chromosome that pair up with each other inside a cell during fertilization. Homologs have the same genes in the same loci, where they provide points along each chromosome that enable a pair of chromosomes to align correctly with each other before separating during meiosis. This is the basis for Mendelian inheritance, which characterizes inheritance patterns of genetic material from an organism to its offspring parent developmental cell at the given time and area.

<span class="mw-page-title-main">Nondisjunction</span> Failure to separate properly during cell division

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division (mitosis/meiosis). There are three forms of nondisjunction: failure of a pair of homologous chromosomes to separate in meiosis I, failure of sister chromatids to separate during meiosis II, and failure of sister chromatids to separate during mitosis. Nondisjunction results in daughter cells with abnormal chromosome numbers (aneuploidy).

<span class="mw-page-title-main">Synapsis</span> Biological phenomenon in meiosis

Synapsis is the pairing of two chromosomes that occurs during meiosis. It allows matching-up of homologous pairs prior to their segregation, and possible chromosomal crossover between them. Synapsis takes place during prophase I of meiosis. When homologous chromosomes synapse, their ends are first attached to the nuclear envelope. These end-membrane complexes then migrate, assisted by the extranuclear cytoskeleton, until matching ends have been paired. Then the intervening regions of the chromosome are brought together, and may be connected by a protein-RNA complex called the synaptonemal complex. During synapsis, autosomes are held together by the synaptonemal complex along their whole length, whereas for sex chromosomes, this only takes place at one end of each chromosome.

<span class="mw-page-title-main">Sister chromatids</span> Two identical copies of a chromosome joined at the centromere

A sister chromatid refers to the identical copies (chromatids) formed by the DNA replication of a chromosome, with both copies joined together by a common centromere. In other words, a sister chromatid may also be said to be 'one-half' of the duplicated chromosome. A pair of sister chromatids is called a dyad. A full set of sister chromatids is created during the synthesis (S) phase of interphase, when all the chromosomes in a cell are replicated. The two sister chromatids are separated from each other into two different cells during mitosis or during the second division of meiosis.

Mitotic recombination is a type of genetic recombination that may occur in somatic cells during their preparation for mitosis in both sexual and asexual organisms. In asexual organisms, the study of mitotic recombination is one way to understand genetic linkage because it is the only source of recombination within an individual. Additionally, mitotic recombination can result in the expression of recessive alleles in an otherwise heterozygous individual. This expression has important implications for the study of tumorigenesis and lethal recessive alleles. Mitotic homologous recombination occurs mainly between sister chromatids subsequent to replication. Inter-sister homologous recombination is ordinarily genetically silent. During mitosis the incidence of recombination between non-sister homologous chromatids is only about 1% of that between sister chromatids.

<span class="mw-page-title-main">Sister chromatid exchange</span>

Sister chromatid exchange (SCE) is the exchange of genetic material between two identical sister chromatids.

<span class="mw-page-title-main">Bivalent (genetics)</span>

A bivalent is one pair of chromosomes in a tetrad. A tetrad is the association of a pair of homologous chromosomes physically held together by at least one DNA crossover. This physical attachment allows for alignment and segregation of the homologous chromosomes in the first meiotic division. In most organisms, each replicated chromosome elicits formation of DNA double-strand breaks during the leptotene phase. These breaks are repaired by homologous recombination, that uses the homologous chromosome as a template for repair. The search for the homologous target, helped by numerous proteins collectively referred as the synaptonemal complex, cause the two homologs to pair, between the leptotene and the pachytene phases of meiosis I.

Chromosome segregation is the process in eukaryotes by which two sister chromatids formed as a consequence of DNA replication, or paired homologous chromosomes, separate from each other and migrate to opposite poles of the nucleus. This segregation process occurs during both mitosis and meiosis. Chromosome segregation also occurs in prokaryotes. However, in contrast to eukaryotic chromosome segregation, replication and segregation are not temporally separated. Instead segregation occurs progressively following replication.

<span class="mw-page-title-main">REC8</span> Protein-coding gene in the species Homo sapiens

Meiotic recombination protein REC8 homolog is a protein that in humans is encoded by the REC8 gene.

<span class="mw-page-title-main">Chiasma (genetics)</span>

In genetics, a chiasma is the point of contact, the physical link, between two (non-sister) chromatids belonging to homologous chromosomes. At a given chiasma, an exchange of genetic material can occur between both chromatids, what is called a chromosomal crossover, but this is much more frequent during meiosis than mitosis. In meiosis, absence of a chiasma generally results in improper chromosomal segregation and aneuploidy.

<i>Goniaea australasiae</i> Species of grasshopper

Goniaea australasiae is a species of grasshopper in the family Acrididae.

<span class="mw-page-title-main">Meiotic recombination checkpoint</span>

The meiotic recombination checkpoint monitors meiotic recombination during meiosis, and blocks the entry into metaphase I if recombination is not efficiently processed.

The origin and function of meiosis are currently not well understood scientifically, and would provide fundamental insight into the evolution of sexual reproduction in eukaryotes. There is no current consensus among biologists on the questions of how sex in eukaryotes arose in evolution, what basic function sexual reproduction serves, and why it is maintained, given the basic two-fold cost of sex. It is clear that it evolved over 1.2 billion years ago, and that almost all species which are descendants of the original sexually reproducing species are still sexual reproducers, including plants, fungi, and animals.

<span class="mw-page-title-main">Neocentromere</span>

Neocentromeres are new centromeres that form at a place on the chromosome that is usually not centromeric. They typically arise due to disruption of the normal centromere. These neocentromeres should not be confused with “knobs”, which were also described as “neocentromeres” in maize in the 1950s. Unlike most normal centromeres, neocentromeres do not contain satellite sequences that are highly repetitive but instead consist of unique sequences. Despite this, most neocentromeres are still able to carry out the functions of normal centromeres in regulating chromosome segregation and inheritance. This raises many questions on what is necessary versus what is sufficient for constituting a centromere.

Holocentric chromosomes are chromosomes that possess multiple kinetochores along their length rather than the single centromere typical of other chromosomes. They were first described in cytogenetic experiments in 1935. Since this first observation, the term holocentric chromosome has referred to chromosomes that: i) lack the primary constriction corresponding to the centromere observed in monocentric chromosomes; and ii) possess multiple kinetochores dispersed along the entire chromosomal axis, such that microtubules bind to the chromosome along its entire length and move broadside to the pole from the metaphase plate. Holocentric chromosomes are also termed holokinetic, because, during cell division, the sister chromatids move apart in parallel and do not form the classical V-shaped figures typical of monocentric chromosomes.

Abby F. Dernburg is a professor of Cell and Developmental Biology at the University of California, Berkeley, an Investigator of the Howard Hughes Medical Institute, and a Faculty Senior Scientist at Lawrence Berkeley National Laboratory.

Ovum quality is the measure of the ability of an oocyte to achieve successful fertilisation. The quality is determined by the maturity of the oocyte and the cells that it comprises, which are susceptible to various factors which impact quality and thus reproductive success. This is of significance as an embryo's development is more heavily reliant on the oocyte in comparison to the sperm.

References

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  10. ↑ Grozeva, Snejana; Nokkala, Seppo; Simov, Nikolay (2009-12-29). "Chiasmate male meiosis in six species of water bugs from infraorders Nepomorpha and Gerromorpha (Insecta: Heteroptera)". Comparative Cytogenetics. 3 (2): 125–130. doi: 10.3897/compcytogen.v3i2.19 . ISSN   1993-078X.
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