Genetic Aspects of Forest Ecosystems

Lecture Notes - FOR 3213

Components of Variation - the phenotype (P) is individual which is determined by the genotype (G) and environment (E).

Some traits are under strong genetic control such as bole straightness, specific gravity of wood, monoterpene levels, leaf rust susceptibility and bud burst and other are under strong environmental control such as height, diameter and seed number.

Plasticity of the Phenotype - phenotypic variation that is nongenetic change due to the environment. Plasticity is to be expected in organisms such as trees which can not move and must make the most of the opportunities provided by the environment.

Experiments to determine the degree of genetic and environmental control of a trait:

A B C

P1 = G1 + E1 P4 = G1 + E1 P7 = G1 + E1

P2 = G2 + E2 P5 = G1 + E2 P8 = G2 + E1

P3 = G3 + E3 P6 = G1 + E3 P9 = G3 + E1

Experiment A shows the typical natural conditions where it is impossible to determine the degree of genetic and environmental control. Experiment B shows a clonal study designed to determine the degree of environmental control by growing a single clone in a variety of environments. Experiment C shows a common garden study to determine the amount of genetic control by growing many genotypes in a single garden.

Causes of plasticity:

- environmental fluctuation shorter than generation time

- environmental changes occur over short distances such as lake margins

- development takes place over a long time, diameter growth

Traits for which plasticity is disfavored

- when constancy is important, seeds, flowers

- when form is determined early in development, leaf margins

Sources of Genetic Variation

- mutation

- recombination

Natural Selection - differential and non random reproduction of genotypes; differential reproduction not differential mortality since some individuals may not reproduce well even though they survive

Evolution

- depends on genetic variability

- basic unit is the population

- without selection, mutation or migration the population will reconstitute itself each generation

Sexual breeding systems - reproduction is (1) opportunity to increase representation of adapted type and (2) way to maintain and produce variability.

Successful species compromise abundance with ability. Abundance refers to rapid reproduction of adapted types and ability refers to production of variation that insures adaptability of the population.

self pollination = good seed production

cross pollination = variability

Selfing is promoted by production of complete flowers or both male and female flowers on the same plant. Outcrossing is promoted by self incompatibility and dioecy. Dioecy is very risky and no annuals are dioecious.

Selfing

1. reliable seed set

2. economical - small flowers, no nectar, less pollen

3. seed set in short growing season

4. rapid establishment of single favored genotype

5. long distance dispersal, establish new population with one seed

6. no inbreeding depression

7. no need for special pollinators

8. re-establishment after disaster is easy

Outcrossing

1. maintenance of heterosis

2. promotion of genetical recombination, fitting into variable environment, resistance to pests, long-term adaptation to changing climate, short-term: loss of highly adapted type

Asexual Systems - Apomixis

- scattered species, rarely a whole genus

- short-term advantage

- relatives are frequently obligate outcrossers

- frequently polyploids which means meiosis disrupted

- a way to fix heterosis

- immediate fitness

Genecology - study of adaptive properties of a sexual population, race, species, subspecies, local inbreeding population; phenotypic variation that is correlated with the environment and is not environmentally induced - it is genetic

Patterns: 1. clinal or continuous and 2. discontinuous

Ecotype - a group within a species that shows genetic variation from other groups within the species that is correlated with environmental variation; can be local such as elevational ecotypes or regional such as latitudinal ecotypes; some species show both local and regional ecotypes

Example - Scots pine which is an extremely wide ranging species shows ecotypic variation for height growth, foliage color, stem form, rooting habit, resistance to insects, fruitfulness, tracheid length and bud set.

Environmental factors eliciting ecological differentiation - temperature, photoperiod, amount and periodicity of rainfall - all very regular and predictable

Photoperiod is a very regular cycle that elicits ecotypic variation on a regional level; controls dormancy in the fall before weather becomes too severe; for wide ranging species growth will usually cease at longer photoperiods for northern races than for southern ones. Elevation can have the same effect as latitude. High elevation sources cease growth at the same photoperiod as sources from more northerly latitudes. In Sweden 1 degree of latitude equals 100 meters of elevation. In the Appalachians 1 degree of latitude equals about 189 meters of altitude.

Local diversity - upland versus lowland types of northern white cedar, north and south slope sources of Douglas-fir were different.

Factors affecting Ecological Differentiation

Gene flow - 1. pollen, 2. seeds 3. other propagules

Factors restricting gene flow - 1. limited number of breeding individuals, 2. differences in flowering time, 3. limited dispersal of pollen and seeds

Synchrony of flowering is more likely in small area. Elevation strongly influences bud break and flowering.

Selection by the environment - disruptive, directional or stabilizing

Isolation mechanisms that may separate species - geographic, ecological, temporal, specific pollination requirement, gametic incompatibility

Polyploidy - three or more sets of chromosomes, most often in high latitude species, uniformity promoting, up to 6 chromosomes for each trait, highly buffered system, superior performance in colonizing some habitats that are relatively uniform; Pinus 2x = 24; Betula 2x = 28( bog birch 4x = 56, yellow birch 6x = 84, paper birch 2x to 6x; Redwood 6x = 66

Fitness versus flexibility

Fitness - inbreeding, apomixis, polyploidy, low rate of recombination

Flexibility - cross pollination, recombination

Populations usually combine both, environment determines the compromise; trembling aspen - fire species, dioecious, root suckering; cottonwood - riparian, dioecious, reproduces by fragmentation along streams