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OCR for page 51
Structure of Genetic
Variation
One of the most fundamental requirements for improved man-
agement of forest tree diversity is understanding the biological
dynamics of genetic variation within and between tree species.
Information about the genetic architecture of tree populations will make
it possible to develop sounder strategies for their conservation and use.
MATING SYSTEMS AND GENE FLOW
Considerable variation exists among tree species with respect to the
extent of genetic diversity and the way such diversity is spatially or
temporally organized within and among populations. The extent and
the pattern of genetic diversity in forest trees are strongly regulated by
their mating patterns and gene flow. Despite decades of research on
the reproductive biology of forest trees, knowledge about the subject is
still limited.
Mating Mechanisms
Mating systems among trees are quite varied. Most tropical forest
trees have hermaphroditic flowers; that is, the flowers contain male and
female reproductive organs (Ashton, 1969; Bawa, 1974, 1979~. In contrast,
a vast majority of temperate tree species are monoecious; that is, they
are characterized by the presence of unisexual flowers, but male and
female flowers occur on the same plant. Dioecy (the presence of separate
male and female plants) occurs in temperate and tropical species, but
51
OCR for page 52
52 / Forest Trees
it is more common in the latter (Bawa and Opler, 1975~. The evolutionary
dynamics of forest trees is further complicated by the existence of mixed
mating and reproductive systems. Selfing (self-pollination or self-fertil-
ization) is possible in hermaphroditic and monoecious species, but it is
usually prevented by a wide variety of genetic mechanisms or differences
in the maturation of male and female floral parts.
Many tropical species are genetically self-incompatible; that is, little
or no seed is set following self-pollination (Bawa, 1974; Bawa et al.,
1985~. Surprisingly little is known about self-incompatibility in temperate
species with hermaphroditic flowers; in the monoecious species, two
mechanisms prevail that prevent or reduce selling. First, the maturation
of male and female flowers at different times, as happens, for example,
in most temperate coniferous trees, reduces the possibility of selling.
Second, self-sterility, which also occurs in most conifers, causes most
selfed seeds to be aborted before they mature. This mortality is assumed
to be due to lethal recessive genes that are brought together when
individuals that are closely related genetically are crossed (inbreeding).
In sum, designing tree reserves and managing natural or artificial
stands of trees requires an understanding of the reproduction of the
trees involved. Without it, tree populations could, for example, fail to
reproduce, or they could experience excessive or unintended inbreeding
and might thus become endangered.
Inbreeding
Various mechanisms can eliminate the possibility of selling, but they
do not exclude inbreeding. The level of inbreeding is determined not
only by the nature of the reproductive system, but also by family
structure, which itself is influenced by pollen and seed dispersal
characteristics. Dispersal of pollen and seeds over a limited area can
increase the genetic relatedness of nearby individuals in a mating
population and, thereby, the potential for inbreeding. The longevity of
some trees over others in the population can also result in a relatively
few individuals genetically dominating the gene pool. This, combined
with such factors as asynchronous flowering of male and female flowers,
overlapping generations within the populations, and unequal sex ratios,
can potentially increase the level of inbreeding. Apomixis, or uniparental
reproduction, which appears to be quite common in some tropical trees
(Ashton, 1988), may also contribute to inbreeding. The net result of
these factors is that the effective population size in terms of reproductive
capacity is often much smaller than the total number of adult trees.
OCR for page 53
Structure of Genetic Variation 153
Outcrossing
Outcrossing refers to the mating of genetically nonidentical individ-
uals. The rate of outcrossing is often estimated by the use of genetic
markers in the form of allozymes (genetic variants of enzymes). In
temperate and tropical trees, outcrossing rates have been found to be
very high. Although the outcrossing rate can range from 0 (no outcross-
ing) to 1 (100 percent outcrossing), it varies from 0.60 to 1 in the tree
species examined by Knowles et al. (1987), and wide variations in rates
occur among individuals even of the same population. The distances
between trees and the timing of reproductive flowering (i.e., spatial
and temporal variation) can also affect outcrossing rates. Rates in
tamarack, for example, have been found to vary with stand density,
the outcrossing rate being lower in less dense than in more dense
stands. A practical implication of this result is that spatial isolation of
trees due to forest decline or degradation may increase the level of
inbreeding and its concomitant effects.
Gene Flow
Gene flow implies an exchange of gametes or genes among dispersed
trees, and it is inversely related to population differentiation. It occurs
through the movement of pollen and seed, which can be dispersed by
a wide variety of nonliving and living mechanisms. Knowing how
populations are expanded, maintained, or restricted by gene flow is
essential for managing distinct tree populations in their original envi-
ronment.
Gene flow determines the geographic scale over which populations
may be differentiated from each other. Population differentiation can
also occur in response to selection resulting from local variations in
environmental factors. Discriminating between selection effects and
migration-induced patterns, however, is difficult. Moreover, selection
or sexually divergent migration, which can be frequent among tree
species, can have significant impact on gene flow (Gregorius and
Namkoong, 1983; Namkoong and Gregorius, 1985~. Where lack of gene
flow occurs, the potential for future development of distinct populations
exists.
All temperate conifers and many hardwood species, such as oaks and
poplars, are wind pollinated. Although most pollen is dispersed by
windfalls near the paternal tree, much is still carried farther away,
several hundred meters or more (Levin and Kerster, 1974~. This, in
OCR for page 54
54 / Forest Trees
combination with the high densities of these temperate, wind-pollinated
species, results in a potentially high frequency of pollen exchange.
Some temperate trees are pollinated by animals (including insects),
but virtually nothing is known about the extent to which animal
pollinators move among trees. In fact, in most instances even the pollen
vectors (carriers) have not been identified. Moreover, in a few instances
in which both bird and insect pollinators are effective, as in Camellia
japonica (Japanese camellia), the relative visitation frequencies of the
pollinators are not known, but it is known that the distances over which
they distribute pollen can vary considerably.
The diversity of insect and other animal pollinators in the tropics is
immense, ranging from bats with a wingspan of 1.5 m to tiny wasps
that are 1 or 2 mm in size. Many species, however, are pollinated by
medium-sized to large bees (Bawa et al., 1985~. Other pollen vectors
are moths, beetles, and the generalist insects, which are the wide variety
of small bees, beetles, butterflies, wasps, and flies that visit flowers.
THE PACKAGING OF GENETIC INFORMATION
A gene, in the classical sense, is the basic unit of heredity and has one
or more specific effects on an organism. Genes are segments of DNA
(deoxyribonucleic acid) in the nucleus of the cell, linearly arranged to
form threadlike structures called chromosomes. The term "locus" refers
to the position of a gene on the chromosome, and it is sometimes used
interchangeably with the term "gene" when referring to regions of DNA
that are influencing a trait.
Alternate forms of a gene found at the same locus are called alleles.
Some genes have many alleles, which allow for multiple gene products
and therefore multiple phenotypes. For example, multiple alleles have
been identified for many of the genes that code for human blood proteins.
The allelic frequency refers to the proportion of loci in the population
occupied by each allele. A gene for which there is more than one common
allele is called polymorphic.
Most organisms are diploid, meaning they carry two copies of each
gene. If both copies are the same allele, the individual is said to be
homozygous; if the two copies are different alleles of the gene, the
individual is heterozygous for that locus.
DNA is the chemical that carries genetic information in all living
organisms. The DNA molecule itself is an elongated double helix, often
compared with a long, twisted ladder. Corresponding to the rungs of the
ladder are two bases, forming a "base pair." The sequence of base pairs
at a given locus confers the specificity required for transmission of
OCR for page 55
Structure of Genetic Variation / 55
Relatively few tree species are pollinated by bats and birds, although
some important exceptions are the durian fruit (Durio) of Asia, the
timber genus Caryocar of the Amazon, many tropical legumes, andspecies
of the Bombacaceae. However, these animals do pollinate a large number
of other species in the forest.
The extent to which pollen vectors move pollen between plants largely
depends on their foraging behavior and the spatiotemporal distribution
of flowers. Insects and other animals may bring about extensive exchange
of pollen among individuals scattered over a wide area (Regal, 1977~.
This is particularly true where specificity (uniqueness) between pollen
vector and the plant is high. An example is the genus Ficus (fig), in
which about 800 species are pollinated by different species of wasps
(Wiebes, 1979~. The high specificity enables the fig trees to reproduce
and persist even in very low densities (lanzen, 1979~. However, this
extreme specificity between pollen vector and plant is not common
among tropical forest trees.
information via the genes. Due to variability of the base pairs, the number
of different alleles that could theoretically be formed from even a very
short piece of DNA is extremely large. For instance, a segment of DNA
with only 10 base pairs could have more than 1 million different codes.
The "average" gene is thought to have up to a thousand base pairs; thus,
the DNA structure provides for a phenomenal amount of variation in the
genetic code. The difference between two alleles is often as simple as a
substitution of a single base pair, but this may correspond to a significant
difference in the phenotype resulting from those alleles.
Variation can readily be observed at several levels: between species,
between major types within a species, between populations within a
major type, and between individuals. An individual's genetic composition,
or genotype, in conjunction with the environment in which that individual
is found naturally, determines the phenotype or observable character
. .
1S lCS.
Certain traits are controlled by a single gene and are referred to as
qualitative or Mendelian traits. Many characteristics, however, are influ-
enced by a larger number of genes; these are called quantitative or
polygenic traits. The cumulative action of these genes influences the
expression of the trait, but the effect of any single gene is small and
cannot generally be isolated in the phenotype. Important production
traits, such as yield, growth rate, and straightness, are examples of
polygenic traits. Occasionally, a "major" gene, one that has a stronger
influence on the trait, can be detected, but there is nevertheless modifi-
cation of the trait by other genes with smaller effects.
~_ ~
OCR for page 56
~ / ~ ~
~ ~ ^~ ~ ^ ~
The Quit of F~f~~ If
has a ~ed-~ntainin~ stone en-
~sed in ~ white sweet pulp that
is att=~cti~ to forge bins and
hats. Credit: Douglas ~13.
Lit~eis known about the distances aver ibich ani~n~alvectors Wave
pollen. Terdiodal binds, such as h~dngLirds; age more sedentary
than nonte~dfor~1 ones, and con~sequentl~v, their!po~l~l~e~n~d~ispersal~}
extra me~lylimi~d (Linha~,1973~. Bats/ on! The other Bia~nd~,Jre known
to ~ awe over a range of seve~~l~kilo~e~ters <~Heiih;~s et~a~l~., 1973)
.mI;a~y/ ~eol~-~s~lzec ~!° large bees can He pope among plants
that am sevefsI~k[~o~ete~ apart (Fr~nkie et<1.,1~976;~J
OCR for page 57
Structure of Genetic Variation / 57
Neutral Alleles and the Quantification of Gene Flow
Alleles are the alternate forms that a gene may take, which may cause
selectively important differences to exist among individuals. Neutral
alleles provide no selective advantage or disadvantage and, therefore,
are useful for modeling gene flow in populations. The flow of neutral
alleles between populations can be quantified by estimating the value
of Nm (N = effective size of the local population; m = average rate of
gene migration by means of pollen and seeds). The principal reason for
estimating Nm is that its value has been shown to indicate the relative
importance of gene flow and genetic drift in explaining the observed
patterns of genetic differentiation in a population (Slatkin, 1987~. If Nm
is less than 1, then changes in allele frequencies resulting from genetic
drift of neutral alleles can occur. Such changes are not likely if Nm is
greater than 1.
As measured by electrophoretically distinguishable alleles, which are
often considered to be neutral variations, the values for Nm in a range
of forest tree species indicate fairly high levels (> 1.0) of gene flow
(Table 3-1~. This is to be expected because most trees have high
outcrossing rates, and there is a positive association between outcrossing
rate and the level of gene flow (Govindaraju, 1988a). Wind-pollinated
species have higher outcrossing rates than animal-pollinated species
(Govindaraju, 1988b). The effects of seed dispersal on levels of gene
flow have not yet been quantified (Hamrick and Loveless, 1987~.
Nonrandom assemblages of genotypes impose a structure on the
TABLE 3-1 Relation of Levels of Gene Flow (Nm) to Pollination
Mechanism in Selected Forest Tree Species
Number of
Populations Pollination
Species Sampled Nma Mechanism'
Abies balsamea 4 4.546 W
A. Iasiocarpa 3 7.300 W
Berthollefia excelsa 2 1.626 A
Eucalyptus caesia 7 0.001 A
ssp. caesia
E.caesia 6 0.174 A
ssp. magma
E. cloesia~a 17 1.081 A
E. delegates is 8 0.606 A
E. oblique 4 0.438 A
(continued)
OCR for page 58
If
TABLE 3-1 s(O~f~
Number of
Populates ~P6llina~S~=
Species ampler \ ~{chanis~'
S. ~# 3 5. ~A
10 ~810
If 3 1.^
, . . .
,. . .
P ~3 1 ~W
~ ~71 3 6 ~W
a, . ~ . . ~ ~ ~ ~. ,~
~41 ~ W
~ ~ ~ ~ - ~ ~ ~
/ ~ a, W
P In 94.62)W
Sap. ~f~-
3. = ~57.~0W
. ·. ..
.,^ . ,^ ~^.,^
at, ,, ;~E; ;/ ~; ~t
1
. ~ ~
a. ~5 4.# ~
P. fife ~Tag W
UP. ~7 0.051 ~
POW
asp. If
P. ~5 1.0 ~W
. , ,
en- ^ Off
:
P. ~ ~6 2.9 ~W
P. ~5 l.p ~W
11 8.#
,
P - '7 ~14 7 483 W
.. go, .....
P ~2 ~Z.780 W
P. ~2 2.~1 TWO
~W
............
P. / ~1~0 3.020 W
6 S~1 W
Q~s~~ 3 2.e5 W
Q. j7 ~3 2.4 ~W
^ .. ~. - ~...
# ~ I ~ ala v~
. ~ ~ , .~ , ~.
.~
a. If 2 3.614 W
Q. r ~2 0.< ~
an. ~a 0 5 ~
Is ~2.1 ~W
~ ~ ~ ~ ~ =
of pollen and If. Valises of \~ less than 1 indicate the potential far genetic dog.
W = mind pll~i~nat~; ^ ~ p~llina~d by an inflect or other animal.
u~cE Adapted from D. R. Govindar~iu. 193. Rel~t~i<~nship Street dispersal ability
and levels of Cane COD in p)an~. Oikos 52:31~5. Repdnted with permission.
OCR for page 59
Structure of Genetic Variation / 59
population, which can be measured as the departure from standard
statistical (Hardy-Weinberg) proportions. Structure can result from
selection and mating among closely related individuals (e.g., siblings
or cross-generational relatives) that are near one another. In studies of
Pin us sylvestris (Scots pine) (Tigerstedt, 1 984) and Liriodendron tulipifera
(tulip tree) (Brotschol et al., 1986), some evidence exists for such
inbreeding or mating within localized populations. Thus, although the
potential exists for both wide intermating and for neutral alleles to be
randomly distributed, there is growing evidence that various genotypes
in plant populations are not randomly distributed.
Nonrandom mating within populations of continuous forest stands
has been reported in several cases: Pinus ponderosa (western yellow or
ponderosa pine) (Linhart et al., 1981a,b), Cryptomeria (red cedar) (Sake)
and Park, 1971), Eucalyptus (eucalypts) (Moran and Hopper, 1987), and
in some tropical species (O'Malley and Bawa, 1987~. A well-documented
example of genetic structuring is a small population of Pinus tueda
(loblolly pine). Allelic differences among groups have been reported in
the continuous population and are assumed to be due to three separate
regeneration episodes that established the current population (Roberds
and Conkle, 1984~. Thus, genetic variations within and among popu-
lations can be differentially distributed, and sampling or collecting that
variation must include those structural variations and not depend on
random methods.
ESTIMATING GENETIC VARIATION
Establishing priorities for the conservation, management, and use of
tree genetic resources requires an understanding of the degree of
diversity among and between specific populations. The scientific meth-
ods used to distinguish levels of variation are the working tools for
shaping decisions on which resources should be devoted to managing
distinct tree characteristics. Two approaches have been used to study
genetic diversity in tree populations: provenance testing and allozyme
screening. The two approaches are used with different objectives in
mind, and the results obtained from each have different applications in
theory and practice.
Provenance (geographic origin) testing is performed by gathering
seeds from different populations and observing variation in performance
(e.g., height, diameter, color, yield) in plants grown under uniform
environmental conditions within one or more planting sites. The pop-
ulations sampled are generally from different geographic and climatic
regions. This approach constitutes the basis of provenance testing in
forestry. Allozyme screening is based on the survey of genetic diversity
OCR for page 60
~ A A
..
as revealed by variation in enzymes atspeciCc gene l!oci..~In general/
own provenance Sting and allozyme screening reve~althat, among
pJ.a!n~ ~
structure of Genetic Variation 161
torreyana and Abies bracteata have restricted ranges, but Pinus resinosa is
widely distributed. Understanding the subtle factors that induce and
maintain genetic diversity provides the scientific foundation for the
optimal management of existing tree genetic variation.
Estunates of Population Genetic Structures
There is growing evidence that various genotypes in plant populations
are not randomly distributed. Nonrandom distribution of genotypes in
a population can make diverse sampling difficult. Selection and mating
among related individuals (siblings or cross-generational relatives) in
close proximity can affect genetic structure and thus sampling within
the population.
For several species, there are more homozygous individuals than
would be expected from random mating, even within plots of 1 ha or
less. This cannot be explained by selfing or other forms of inbreeding.
In one such species, Pinus sylvestris, the excess of homozygotes could
be due to very localized and specific male effectiveness (Tigerstedt,
1984~. This appears to be true for Liriodendron tulipifera (Brotschol, 1983)
but only at the seedling stage, and it is not evident in the adult trees.
Such effects appear to be even more pronounced in some tropical tree
species, probably due to more limited pollen dispersal by animals
(O'Malley and Bawa, 1987~. Pollen flow in these species appears to be
very extensive, but the pollen distribution of individual trees may be
highly localized.
The net effect is that seed and seedling populations may be highly
structured. The implication for sampling is that unless the sampling is
spread over a wide area within the population, the collected seeds may
represent a biased sample of genetic diversity within the population.
Variation Among Populations
Just as variation in species has implications for management, so does
the distribution of that variation through the species. Forest tree species
differ greatly with respect to genetic divergence among populations.
There are two major problems in comparing levels of genetic differen-
tiation within a species. First is the problem of scale; in some species,
sampled populations are separated by several hundred kilometers, in
others by only a few kilometers. Second, genetic divergence in some
species has been studied on the basis of variation in morphological
traits and in others by estimating variation in allozymes. The results
obtained from the two approaches frequently do not agree.
In many species, the evidence for genetic divergence is largely based
on allelic diversity as revealed by allozyme studies. Several methods
