population genetics

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Homozygosity and patch structure in plant
populations as a result of nearest-neighbor
pollination
(inbreeding/population structure/isolation
by distance)
MONTE E. TEURNER, J. CLAIBORNE STEPHENS, AND
WAYATT W. ANDERSON
Department of Molecular and Population Genetics,
University of Georgia, Athens, Georgia 30602
PRESENTED BY- MOHAMMED.N.ABBO.
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INTRODUCTION -
G

• enetic differentiation of populations
  over the habitats occupy a major factor in
  the processes of adaptation and evolution.
  even large populations distributed continuously
  over an area will differentiate if gene
  dispersal within them is sufficiently
  restricted. this biological phenomenon termed
• 
  (Isolation by Distance)
• Sewall Wright et.al 1978.

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In contrast Many genetic characteristics
of such continuous populations depend on the size
of local breeding units or neighborhoods, within
them . furthermore these neighborhoods
are essentially subdivisions created by limited
gene dispersal.
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Neighborhood Size Wright defined a
neighborhood sizes "the number of individuals in
an area from which parents of central
individuals may be treated as if drawn at
random." neighborhood size is important because
it governs the differentiation that results from
short-range dispersal of genes in continuous
populations. One the other hand neighborhood
size was a simple function of the variance in the
distance genes move from parents to offspring.
Two forms of NNP were simulated, and calculated
the neighborhood sizes.
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• THE MODEL

The genetic system consisted of two
alleles at a single locus in a
self-incompatible plant that mates by random
pollen transfer from a neighboring individual.
A computer program had been used to
simulate the population genetics of an
annual plant species visited by
pollinators whose flights are
predominantly between nearest neighbors
. Nonetheless Two alleles at a
single locus composed the genetic system.
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• The population of self incompatible,
  bisexual diploid plants was uniformly
  distributed on the intersection points of a
  100 x 100 grid.
• However flowering and reproduction of
  all individuals in the population were
  synchronized, and generations were non
  overlapping, and size remained a constant
  10,000.

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( Figure 1) -
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Pollinators
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• A male parent had been selected from plants
  neighboring the female parent in one of the two
  ways diagrammed in Fig. 2 With strict NNP the
  four nearest plants on the grid have equal
  probabilities (P 0.25) of serving as male
  parent. With relaxed NNP the 12nearest neighbors
  of the female parent have probabilities of
  serving as male parent according to their
  distance from the female parent.
• There are two ways this latter case can be
  interpreted biologically. Pollinators can move
  to plants which are
• first, second, and third nearest to the maternal
  parent with the probabilities given.
  Alternatively, pollinators could move to
  nearest-neighboring plants only, carrying mostly
  pollen from the last plant visited.

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( Figure 2) -
Mating scheme

• STRICT NNP

• RELAXED NNP

D
C
C
B
A
D
B
B
x
D
A
A
x
C
C
B
A
D
(Left) Strict
(Right) Relaxed X is the
female
A,B,C has probability 0.2125, 0.025, 0.0125 A
probability of 0.25 of being
respectively of being male parent male parent
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• Computer runs were made with three choices of
  gene frequencies P 0.5, P 0.8, and P 0.9
• For each with both strict and relaxed NNP (Fig.
  2).
• Ten replicate populations were simulated for each
  choice of gene frequency and neighborhood size,
  utilizing a different sequence of pseudorandom
  numbers for the chance events of pollination and
  seed dispersal.
• In specified generations the entire population
  was represented visually with a different symbol
  for each genotype.

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Inbreeding and Homozygosity.
The inbreeding effect of NNP was measured by the
coefficient of inbreeding, F. calculated as
simply the proportional loss of
heterozygosity from- (Hardy-Weinberg
expectation) F (Expected heterozygosity -
Observed heterozygosity)
Expected heterozygosity.
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Values of inbreeding coefficient F

1. F in a single population over 720 generations.
2. Mean F in sets of 10 replicates begun at each
   of three gene freq.
3. Mean F in sets of 10 replicates with strict or
   relaxed NNP.

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F-Coefficients
Combine different sources of reduction
in expected heterozygosity into one equation
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• Beginning with a random distribution of
  genotypes, small patches of black or grey
  homozygotes appear within five generations and
  grow steadily with time (Fig.3).
• Nevertheless proportion of homozygotes in
  the population continues to grow, and the
  black and grey patches consequently expand in
  area. This rapid increase in homozygosity
  continues for about 25 generations and accounts
  for the initial are increased .
• Nonetheless most heterozygotes are found at
  the borders between patches of the two
  homozygotes, although a few are contained
  within the large homozygous patches
  coalesce and emerged by generation 100 there
  are very large patches.

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Isolation by Distance Simulation (Figure 3)

• Population was begun at po.5 with genotypes in
  Hardy-Weinberg frequencies randomly distributed
  in population
• Heterozygotes white rectangles.
• Homozygotes black and grey rectangles

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Conclusions

• Inbreeding reduce heterozygosity and increase
  homozygosity in populations, we implicate that
  in our endeavors in scientific fields and
  applications .
• The genotypic patches which evolved simulations
  of plant species breeding under NNP have
  counterparts in nature.

• The computer simulations are particularly
  fruitful in providing a visual image of the
  spatial relationships of genotypes in a large
  population,
• Merely can furnish that like a picture of
  geographic patterning that would be difficult to
  deduce from the mathematical theory alone.

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Thank you