Mating systems in forest trees can involve outcrossing, self-fertilization, or asexual reproduction. Outcrossing, where genetically different individuals mate, is promoted by mechanisms like dichogamy, protandry, protogyny, herkogamy, and self-incompatibility. The document discusses these mechanisms and describes the primary mating systems of outcrossing, autogamy, apomixis, and mixed systems in plants, as well as factors that influence outcrossing rates in tree species.

1. Mating system Dynamics in Forest
Trees-Factors affecting-Mechanisms
promoting high levels of Out crossing
R.SARANYA KUMARI
20195626004
1st M.Sc., Forestry (FBT)

2. Mating design
 A mating system is a way in which a group is structured in
relation to sexual behaviour. It refers to the degree and
circumstances of out crossing.
 Variation exists among tree species with respect to the
extent of genetic diversity
 The extent and the pattern of genetic diversity in forest
trees are strongly regulated by their matting patterns and
gene flow.

3. The primary mating systems in plants are
 Out crossing (cross-fertilization)
 Autogamy (self-fertilization)
 Apomixis (asexual reproduction without fertilization,but
only when it arising by modification of sexual function)
 Mixed mating systems, in which plants use two or even
all three mating systems.

4.  Number of models have been used to estimate the
parameters of plant mating systems.
 The basic model is the mixed mating model, which is based
on the assumption that every fertilization is either self-
fertilization or completely random cross-fertilization.
 Effective selfing model recognizes that mating may be more
common between pairs of closely related plants that
between pairs of distantly related plants.

5. Mating mechanisms
 Mating system among trees are quite varied. Most tropical
forest trees have hermaphroditic flowers; that is, the
flowers contain male and female reproductive organs.
 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 occurs in temperate and tropical
species.
 The evolutionary dynamics of forest trees is further
complicated by the existence of mixed mating and
reproductive systems.

6.  Selfing (Self pollination or self fertilization) is possible in
hermaphroditic and monoecious species.
 Many tropical species are genetically self-incompatible.
 In monoecious species, two mechanisms prevail that prevent or
reduce selfing.
[1]. The maturation of male and female flower at different
time, for example, in most temperate coniferous trees, reduces
the possibility of selfing.

7. [2]. 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 individuals that are closely
related genetically are crossed (inbreeding).

8. Out crossing
 Out crossing refers to the mating of genetically non identical
individuals.
 The rate of out crossing is often estimated by the use of
genetic markers in the form of allozymes (genetic variants of
enzymes).
 In temperate and tropical trees, out crossing rates have been
found to be very high.
 The out crossing rate can range from 0 (no out crossing) to 1
(100 % out crossing),it varies from 0.60 to 1 in the tree
species

9. Cont….
 The distances between trees and the timing of reproductive
flowering (i.e., spatial and temporal variation) can also affect
out crossing rates .
 Out crossing rates also vary with stand density, the out
crossing 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.

10. Autogamy
 Self-fertilization (also known as autogamy) occurs in
hermaphroditic organisms where the two gametes fused in
fertilization come from the same individual.
 They are bound and all the cells merge to form one new
gamete.
1. Bisexuality
2. Homogamy
3. Cleistogamy
4. Chasmogamy
5. Position of anthers
6. Geitogamy

11. 1.Bisexuality
Presence of male and female organs in the same flower
is known as bisexuality. The presence of bisexual flowers
is a must for self pollination. All the self pollinated plants
have hermaphrodite flowers.
2. Homogamy
Maturation of anthers and stigma of a flower at the same
time is called homogamy. As a rule homogamy is essential
for self pollination.

12. 3. Cleistogamy
When pollination and fertilization occur in unopened flower
bud, it is known as cleistogamy. Cleistogamy has been
reported in some grass species.
4. Chasmogamy
Opening of flowers only after the completion of
pollination is known as chasmogamy.
5. Position of anthers
In some species stigma are surrounded by anthers in such a
way that self pollination is ensured

13. In some legumes, the stamens and stigma are enclosed by the
petals in such a way that self pollination occurs.
6. Geitogamy:
It is the mode of selfing which involves transfer of pollen
between the flowers of the same tree and requires a pollen
agent.
 A certain degree of geitogamy is inevitable in self compatible
mass flowering in the species.
E.g.: Eucalyptus tereticornis, Albizzia lebbeck, and Tectona
grandis.

14. Apomixis
 Apomixis was defined by Hans Winkler as replacement of
the normal sexual reproduction by asexual reproduction,
without fertilization.
 Apomictically produced offspring are genetically identical to
the parent plant.
 In flowering plants, the term "apomixis" is commonly used
in a restricted sense to mean agamospermy, i.e. asexual
reproduction through seeds.

15. Apomixis in flowering plants
 Agamospermy, asexual reproduction through seeds, occurs in
flowering plants through many different mechanisms.
Agamospermy occurs mainly in two forms:
1. Gametophytic apomixis
2. Adventitious embryony

16. Cont…..
 In gametophytic apomixis, the embryo arises from an
unfertilized egg cell (i.e. by parthenogenesis) in a
gametophyte that was produced from a cell that did not
complete meiosis.
 In adventitious embryony (sporophytic apomixis), an
embryo is formed directly (not from a gametophyte) from
nucellus or integument tissue.

17. Types of apomixis in flowering plants
1. Nonrecurrent apomixis
2. Recurrent apomixis (also called gametophytic apomixis)
3. Vegetative apomixis

18. Non recurrent apomixis
In this type "the megaspore mother cell undergoes the usual
meiotic divisions and a haploid embryo sac is formed.
 The new embryo may then arise either from the egg (haploid
parthenogenesis) or from some other cell of the gametophyte
(haploid apogamy).
 The haploid plants have half as many chromosomes as the
mother plant, and "the process is not repeated from one
generation to another also parthenogenesis and apogamy.

19. Recurrent apomixis (gametophytic apomixis)
 In this type, the embryo sac has the same number of
chromosomes as the mother plant because meiosis was not
completed.
 Adventive embryony also called sporophytic apomixis,
sporophytic budding, or nucellar embryony. Here there
may be an embryo sac (gametophyte) in the ovule, but the
embryos do not arise from the cells of the gametophyte.

20. Vegetative apomixis
In this type "the flowers are replaced by bulbils or other
vegetative propagules which frequently germinate while still
on the plant".
 The most complex of these types of apomixis in flowering
plants is recurrent apomixis, now more often called
gametophytic apomixis.
 It is divided into diplospory (generative apospory) in which
the embryo sac arises from a cell of the archesporium, and
apospory (somatic apospory) in which the embryo sac arises
from some other nucellus cell.

21. Male apomixis in a conifer
 A unique example of male apomixis as the regular
reproductive method has recently been discovered in the
Saharan Cypress, Cupressus dupreziana, where the seeds
are derived entirely from the pollen with no genetic
contribution from the female "parent".

22. Mechanisms promotes out
crossing
1.Dichogamy: Separation of anther dehiscence and stigma
receptivity in time.
2.Protoandry: Release of pollen proceeding stigma
receptivity.
3.Protogyny: Stigma becoming receptive before anther
dehiscence.

23. 4.Herkogamy
Hinderance to self pollination due to some physical
barrier such as presence of hyline membrane around the
anther is known as herkogamy.
 Hyline membrane does not allow the dehiscence of pollen and
prevents self pollination.
5.Self incompatibility
Self pollen either may not adhere to the stigma, adhere
but do not germinate or germinate but are unable to penetrate
or grow down the style.

24. 6. Male sterility : Refers to either absence of pollen grain or if
present it is non-functional.
Features of Male Sterility
 Prevents self pollination, permits cross pollination.
 Leads to heterozygosity
 Female gametes function normally
 Assayed through staining techniques
 In nature, occur due to spontaneous mutations
 Can be induced artificially