Editing Chromosomal crossover (section)

===Chiasma===
Chiasma (plural: chiasmata) are essential for the correct alignment and segregation of homologous chromosomes at meiosis I, and their frequency reflects the rate of genetic recombination, which contributes to variation among offspring. Chiasma number tends to increase with genome size, as larger genomes generally undergo more crossover events per meiosis.<ref>{{Cite journal |last=Ross-Ibarra |first=J. |date=March 2007 |title=Genome size and recombination in angiosperms: a second look |url=https://academic.oup.com/jeb/article/20/2/800-806/7324572 |journal=Journal of Evolutionary Biology |language=en |volume=20 |issue=2 |pages=800–806 |doi=10.1111/j.1420-9101.2006.01275.x |pmid=17305845 |issn=1010-061X}}</ref> Each homologous pair forms a [[Bivalent (genetics)|bivalent]] (or tetrad), consisting of four chromatids. The number and position of chiasmata influence the shape of bivalents, rod-shaped with one chiasma and ring-shaped with two or more.<ref>{{Cite journal |last1=López |first1=E. |last2=Pradillo |first2=M. |last3=Oliver |first3=C. |last4=Romero |first4=C. |last5=Cuñado |first5=N. |last6=Santos |first6=J. L. |date=January 2012 |title=Looking for natural variation in chiasma frequency in Arabidopsis thaliana |url=https://academic.oup.com/jxb/article-lookup/doi/10.1093/jxb/err319 |journal=Journal of Experimental Botany |language=en |volume=63 |issue=2 |pages=887–894 |doi=10.1093/jxb/err319 |issn=1460-2431}}</ref><ref>{{Cite journal |last1=Zickler |first1=Denise |last2=Kleckner |first2=Nancy |date=2015-05-18 |title=Recombination, Pairing, and Synapsis of Homologs during Meiosis |url=https://doi.org/10.1101/cshperspect.a016626 |journal=Cold Spring Harbor Perspectives in Biology |volume=7 |issue=6 |pages=a016626 |doi=10.1101/cshperspect.a016626 |pmid=25986558 |issn=1943-0264|pmc=4448610 }}</ref> 

The grasshopper ''[[Melanoplus femurrubrum|Melanoplus femur-rubrum]]'' was exposed to an acute dose of [[X-ray]]s during each individual stage of [[meiosis]], and [[Chiasma (genetics)|chiasma]] frequency was measured.<ref name="pmid5797806">{{cite journal | vauthors = Church K, Wimber DE | title = Meiosis in the grasshopper: chiasma frequency after elevated temperature and x-rays | journal = Canadian Journal of Genetics and Cytology | volume = 11 | issue = 1 | pages = 209–216 | date = March 1969 | pmid = 5797806 | doi = 10.1139/g69-025 }}</ref>  Irradiation during the [[Leptotene stage|leptotene]]-[[Meiosis#zygotene|zygotene]] stages of [[meiosis]] (that is, prior to the [[Meiosis#Pachytene|pachytene]] period in which crossover recombination occurs) was found to increase subsequent chiasma frequency.  Similarly, in the grasshopper ''[[Chorthippus brunneus]]'', exposure to X-irradiation during the zygotene-early pachytene stages caused a significant increase in mean cell chiasma frequency.<ref name="pmid5289295">{{cite journal | vauthors = Westerman M | title = The effect of x-irradiation on chiasma frequency in Chorthippus brunneus | journal = Heredity | volume = 27 | issue = 1 | pages = 83–91 | date = August 1971 | pmid = 5289295 | doi = 10.1038/hdy.1971.73 | doi-access = free | bibcode = 1971Hered..27...83W }}</ref>  Chiasma frequency was scored at the later [[Meiosis#diplotene|diplotene-diakinesis]] stages of meiosis.  These results show that [[Ionizing radiation|ionising-radiation]] induced [[DNA repair|double-stranded DNA breaks]] were subsequently repaired by a crossover pathway leading to chiasma formation.<ref>{{Cite journal |last1=Szostak |first1=Jack W. |last2=Orr-Weaver |first2=Terry L. |last3=Rothstein |first3=Rodney J. |last4=Stahl |first4=Franklin W. |date=May 1983 |title=The double-strand-break repair model for recombination |url=https://linkinghub.elsevier.com/retrieve/pii/0092867483903318 |journal=Cell |language=en |volume=33 |issue=1 |pages=25–35 |doi=10.1016/0092-8674(83)90331-8}}</ref>

However, increased crossover frequency following radiation-induced DNA damage does not universally occur in all insects; for example, ''Drosophila'' females exhibit predominantly non-crossover repair pathways when responding to induced double-stranded DNA breaks, resulting in a relatively low ratio of crossovers to non-crossovers.<ref>{{Cite journal |last1=Mehrotra |first1=S. |last2=McKim |first2=K. S. |date=2006 |title=Temporal Analysis of Meiotic DNA Double-Strand Break Formation and Repair in Drosophila Females |journal=PLOS Genetics |language=en |volume=2 |issue=11 |pages=e200 |doi=10.1371/journal.pgen.0020200 |doi-access=free |issn=1553-7390 |pmc=1657055 |pmid=17166055}}</ref>