Genetic diversity is the backbone of evolution !

Why do we need diversity?The large picture

• Political demand for diversity

• Man’s activities may lead to extinction of populations and species

• Man’s influence seems to grow

• Man became most important ecological factor 50000 years ago

• Diversity is the basis for evolution and thus life

But moas couldn't stand up to human huntingHumans first reached New Zealand by canoe about 1000 years ago. These settlers quickly learned to hunt the defenceless moas. In fact, they learned too well--in only about 700 years, every last moa was gone.

Man destroys biodiversityNot long ago, giant wingless birds like this moa paced New Zealand's open country and brushy woodlands. Without large native predators to threaten them, moas evolved to be the biggest land predators on their island home.

“Biodiversity” is appealing

Evolution

• Diversity lost by selection and chance• Diversity created by mutation• Balance, population size 500 probably enough for

sustainable diversity• Forests much larger than that• Forest tree breeding often operates with numbers

larger than that (>500)

Numeric example ‘The more A, the better' Ranking for AAA**AA**AA 6A**AA*A***A 4A**AAAA**** 4AA*****A*** 2A*A*****A** 2AA***A***** 2A*A**A***** 2A********A* 1AThe three top ranking has together only 8 AIf all are selected the potential for 10 A remainsSelecting best may mean missing something!

Variance in sample

A sample need not be large to preserve the variance of the original population!

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Sample Size

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Rare allelesWhat alleles are rare? Almost all rare alleles are rather recent mutations and of no

value; Little recent advantage; Most rare alleles in our forest trees will die away naturally.

Even if they are of potential value, it is not easy to utilise rare alleles

many generations to get effect;

in few initial genomes inbreeding problems if utilised,

expensive screening to identify.

A gene with a frequency of 10% is likely to be conserved but a gene with frequency of 2% is likely to be lost in a random mating population of size 20.

Mechanisms for important rare alleles:

balanced polymorphism?

sometimes a little bad, sometimes very good?

better if in low frequency.

Limited impact of breeding

• Areas never regenerated on purpose,

• ”Natural” regeneration 25%,

• Natural ”volunteers in plantations,

• Pollen contamination (50% of pollen) in seed orchards.

Unimproved

Improved

Diversity in a stand

  Genetic diversity in a stand is likely to favour biological production:

• A single genotype demands the same things at the same time, bad site use!

• In a mix another genotype may take over the ecological space left by a failed genotype.

• A disease spreads faster in a uniform crop.

This expectation has generally been confirmed by a number of experiments with different agricultural crops

Too much diversity in plantations?!

Most crop- and many forest managers do not like diversity,

• Uniform trees = better economy, simpler management - even if biological production is lost,

• Genetic superiority of the best clones is much larger than the loss in biomass production by uniformity,

• The current demand for diversity in intensively managed forests is – in my opinion - unreasonable expensive in lost future gain,

• Most of the benefits with diversity is obtained by five genotypes instead of an infinite number.

Seed orchard crops are diverse!•Phenotypic selection of plus trees are uncertain, preserves diversity,

•Marker gene measurements indicate that seed orchard crops can be more diverse than stand seeds!

•Seed orchard clones are recruited from a large area, which favours diversity compared to stands,

•Pollen sources outside the seed orchard favours diversity,

•Variances depends little on clonal number,

•In a small piece of a natural forest, trees are as related as trees from a seed orchard plantation.

Measured with marker genes, most of the diversity is within stands and little between stands, table from (El-Kassaby 1991). 

GST gives the share of the genetic diversity that falls between stands.

Species GST (%) Reference

P sylvestris 3.0 Rudin et al 1974

  16.0 Mejnartowitz 1979

  2.0 Gullberg et al 1985

P abies 4.0 Bergman 1974

  5.0 Tigerstedt 1974

  2.0 Lundkvist och Rudin 1977

  3.0 Lundkvist 1979

Reasons to consider gene diversity in breeding

• To boost breeding value when breeding population is creamed for production population,

• To obtain a production population with little inbreeding,

• To offer desirable gene diversity for production population (acceptability and production),

• To allow more aggressive breeding in the first cycles,

• To consume while accumulating additive effects,

• To keep inbreeding manageable in the breeding stock,

• To be prepared for changed emphasises,

• To combine breeding and gene conservation,

• Some intuitive feeling that diversity could be good to have,

• To demonstrate that breeders care for sustainability, breeding is not mining,

• A sense of respect,

• Genetic diversity is the basis for the professions forest geneticist and tree breeder.

Too much diversity in breeding population?!

• Expensive• In conflict with gain

Extremest value increases slowly with number

The most extreme value of a population is raising extremely slow as a function of population size, thus to increase numbers is rather unefficient!

Artificial selection is not a major short time threat to diversity

• Low heritability = conserved variance also after intensive phenotypic selection (gets worse when selection is on genotype instead of phenotype),

• 35000+ genes interact to produce a phenotype. The influence of selection on the frequency of each gene must be small …,

• Selection for quantitative traits are likely to be selection for different genes at different selection occasions.

Need of genetic diversity measure• Demonstration that we care,

• For chiefs and "politicians" to demonstrate that they care,

• To monitor operations,

• To control operations,

• Review and compare different options for operations,

• Classify old and new forests, multiplication units, programs, development by time, diversity on stand, landscape, region and national level,

• Trade off with other quantifiable variables in the breeding system, like genetic gain, cost and time,

• Use for gene conservation purposes.

Group coancestry is the probability that two genes taken at random from a population origin from the same gene in a common ancestor (genomsnittligt släktskap)

Coancestry

Coancestry is the probability that genes taken at random from each of a pair of individuals origin from the same gene in a common ancestor.

Group coancestry• Average coancestry including self-coancestry

(“genomsnittligt släktskap”)• Loss of gene diversity = group coancestry• Group coancestry is a measure of gene diversity!

mother

sister

aunt

uncle

cousin

What is the group coancestry of this ”family”?

Group coancestry and status number are useful as diversity measures!

We need measures to control accumulation of relatedness

Gene conservation can be said to keep group coancestry low

Breeding should combine a high gain and a reasonable group coancestry

or

Breeding is much the art of balancing gain and group coancestry

Interaction: conservation and breeding

Diversity changes at generation shifts because:

1) Drift (unavoidable),

2) Balance of founder genes (can be optimised).