How sex cells get the right genetic mix - An interdisciplinary approach
solves a century-old puzzle

Date:
August 3, 2021
Source:
John Innes Centre
Summary:
A new discovery explains what determines the number and position
of genetic exchanges that occur in sex cells, such as pollen and
eggs in plants, or sperm and eggs in humans.



FULL STORY ==========================================================================
A new discovery explains what determines the number and position of
genetic exchanges that occur in sex cells, such as pollen and eggs in
plants, or sperm and eggs in humans.


==========================================================================
When sex cells are produced by a special cell division called meiosis, chromosomes exchange large segments of DNA. This ensures that each new
cell has a unique genetic makeup and explains why, with the exception
of identical twins, no two siblings are ever completely genetically
alike. These exchanges of DNA, or crossovers, are essential for generating genetic diversity, the driving force for evolution, and their frequency
and position along chromosomes are tightly controlled.

Co-first author of the study Dr Chris Morgan explains the significance
of this phenomenon: "Crossover positioning has important implications
for evolution, fertility and selective breeding. By understanding the mechanisms that drive crossover positioning we are more likely to be
able to uncover methods to modify crossover positioning to improve
current plant and animal breeding technologies." Despite over a
century of research, the cellular mechanism that determines where,
and how many, crossovers form has remained mostly mysterious, a puzzle
that has fascinated and frustrated many eminent scientists. The phrase "crossover interference" was coined in 1915 and describes the observation
that when a crossover occurs at one location on a chromosome, it inhibits
the formation of crossovers nearby.

Using a cutting-edge combination of mathematical modelling and '3D-SIM'
super- resolution microscopy, a team of John Innes Centre researchers
have solved this century old mystery by identifying a mechanism which
ensures that crossover numbers and positions are 'just right': not too
many, not too few and not too close together.

The team studied the behavior of a protein called HEI10 which plays
an integral role in crossover formation in meiosis. Super-resolution
microscopy revealed that HEI10 proteins cluster along chromosomes,
initially forming lots of small groups. However, as time passes, the HEI10 proteins concentrate in only a small number of much larger clusters which,
once they reach a critical mass, can trigger crossover formation.

These measurements were then compared against a mathematical model which simulates this clustering, based on diffusion of the HEI10 molecules and
simple rules for their clustering. The mathematical model was capable
of explaining and predicting many experimental observations, including
that crossover frequency could be reliably modified by simply altering
the amount HEI10.

Co-first author Dr John Fozard explains: "Our study shows that data from
super- resolution images of Arabidopsis reproductive cells is consistent
with a mathematical 'diffusion-mediated coarsening' model for crossover patterning in Arabidopsis. The model helps us understand the patterning
of crossovers along meiotic chromosomes." The work builds on the John
Innes Centre legacy of using plants as model organisms to study conserved
and fundamental aspects of genetics. This same process was also studied
by JIC alumni J.B.S Haldane and Cyril Darlington in the 1930s. The model
also supports predictions that were made by another famous JIC alumnus,
Robin Holliday, in the 1970s.

Corresponding author, Professor Martin Howard, adds: "This work is a great example of interdisciplinary research, where cutting-edge experiments
and mathematical modelling were both needed to unlock the heart of
the mechanism.

One exciting future avenue will be to assess whether our model can
successfully explain crossover patterning in other diverse organisms."
This research will be particularly valuable for cereal crops, such as
wheat, in which crossovers are mostly restricted to specific regions of
the chromosomes, preventing the full genetic potential of these plants
from being available to plant breeders.

========================================================================== Story Source: Materials provided by John_Innes_Centre. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Chris Morgan, John A. Fozard, Matthew Hartley, Ian R. Henderson,
Kirsten
Bomblies, Martin Howard. Diffusion-mediated HEI10 coarsening
can explain meiotic crossover positioning in Arabidopsis. Nature
Communications, 2021; 12 (1) DOI: 10.1038/s41467-021-24827-w ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/08/210803084901.htm

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