Why are chiasmata important

Climate Change 5: Evolution 1. Evolution Evidence 2. Natural Selection 3. Classification 4. Cladistics 6: Human Physiology 1. Digestion 2. The Blood System 3.

Disease Defences 4. Gas Exchange 5. Homeostasis Higher Level 7: Nucleic Acids 1. DNA Structure 2. Transcription 3. Translation 8: Metabolism 1. Metabolism 2.

Cell Respiration 3. Photosynthesis 9: Plant Biology 1. Xylem Transport 2. Phloem Transport 3. Plant Growth 4. Plant Reproduction Genetics 1. Meiosis 2. Inheritance 3. Speciation Animal Physiology 1. Antibody Production 2. Movement 3. The Kidney 4. Sidebar [Skip]. Chiasmata Previous. Because pairing is restricted to chromosome ends, telomeres emerge as the exclusive sites hot spots of both pairing and recombination. It would be immensely rewarding to examine the nature of meiotic chromosome pairing in the ring in the light of the models described by Barzel and Kupiec 1.

In eukaryotes, HR is credited with two important functions during meiosis: it promotes genetic diversity through allele shuffling and also ensures proper chromosome segregation by forming chiasmata 4. At least one chiasma per bivalent is necessary to allow segregation of homologues to opposite poles during MI 5 , 6.

Regular formation of a chromosome ring implies that telomeric regions in evening primrose are competent to sustain robust pairing and HR events, leading to proper MI chromosome segregation. Studying these events would shed light on how end-to-end pairing is regulated to ensure that parental chromosomes are positioned alternately in the ring and also segregated to opposite poles. From a genetics perspective, sex is synonymous with meiotic HR 7.

Despite their sequence diversity 8 , plant telomeres are not known to possess genes. The crossovers that occur in telomeric regions of evening primrose are therefore neutral, in that they do not generate genetic diversity. Lack of genetic recombination might lead to the accumulation of deleterious mutations over generations Muller's ratchet; see Ref. How the ring formers deal with these unique situations would be an interesting focus for evolutionary genetics.

The novel format in which crossover takes place in evening primrose eliminates the genetic and evolutionary consequences of allele shuffling, but retains its mechanistic functions for chromosome segregation through chiasma. Meiosis as a process may have survived, but without generating genetic diversity. Consequent production of 'clones through meiosis' effectively converts sexual reproduction into an asexual one, lending support to the 'twofold cost of sex' hypothesis see Ref.

Molecular dissection of the telomeres of evening primrose and other ring formers, using an 'omics' approach, should enrich our understanding of telomeres and also illuminate the poorly defined but fascinating complexities of meiotic pairing and HR. Barzel, A. Finding a match: how do homologous sequences get together for recombination? Nature Rev. Levy, M. Genic heterozygosity and variation in permanent translocation heterozygotes of the Oenothera biennis complex.

Genetics 79 , — Cleland, R. Oenothera: Cytogenetics and Evolution. Academic, London, Google Scholar. Cromie, G. Branching out: meiotic recombination and its regulation. Trends Cell Biol. Mezard, C. The road to crossovers: plants have their say. Trends Genet. Nicklas, R. Chromosome segregation mechanisms. Genetics 78 , — Hillis, D. Asexual evolution: can species exist without sex?

Lamb, J. Han, F. Plant chromosomes from end to end: telomeres, heterochromatin and centromeres.