Evoluton of plant sexual diversity

WELCOME
18-05-2021 Dept. of Genetics & Plant Breeding 1

The Evolution of Plant
Sexual Diversity
MARUTHI PRASAD B. P.
PGS19AGR8134
Sr. MSc. (Agri.)
Dept. of Genetics & Plant Breeding

Introduction
1
Strategies promoting Cross Pollination
3
Brief History 2
Evolutionary Transitions 4
Conclusion
5
3
Flow of Seminar

What is sexual diversity?

……But why should you care?
Relevant to many applied areas :Plant
breeding and biotechnology, horticulture,
conservation and invasion biology
Exhibit greater variety than equivalent
reproductive structures of any other group of
organisms
Provide outstanding examples of evolution
and adaptation, which can be studied in the
wild

Distinctive features of plants that
influences their sex lives
Immobility
Hermaphroditism
I don’t have
ability to walk
to find a
partner
 Immobility and
hermaphroditism are
highly correlated traits
 Hermaphrodites are often
immobile even in animals
also like corals

Clonality
Modularity and multiple
reproductive structures
Life-history diversity
Distinctive features of plants that
influences their sex lives

How to study plant sex

Plant sex can also be
studied at home in the
Darwinian tradition

Studying plant sex at the DNA level
DNA sequences allow differences between
individuals in single nucleotides to be
identified
Single nucleotide polymorphism (SNP’s) can
be measured for thousand of genes
The consequences of different mating patterns
on genetic diversity can be measured and
genetic relationships among populations
determined

Plant reproductive modes
Asexual Sexual
Dioecious Hermaphrodites
Cross-fertilization Self-fertilization
Reproductive
Systems
Sexual system
Mating system

Why do reproductive organs of
flowers exhibit such astonishing
diversity?

History
• Egyptians crossed male
and female trees of the
date palm to produce
fruits.

The early naturalists began to interpret
floral function and make controlled cross
and self-pollinations
Joseph Koelreuter Thomas Andrew Knight Christian Konrad
Sprengel

Charles Darwin (1809 -1882)
• Naturalist, Scientist
• Father of Evolution
 “On the various contrivances by which British and foreign
orchids are fertilised by insects and on the good effects of
intercrossing”
 “The effects of cross and self fertilization in the vegetable
kingdom”
“The different forms of flowers on plants of the same
species”

Charles Darwin’s three books
provide the foundations for
studies on plant sexual
diversity
1862 1876 1877

Cross pollination Self pollination

• If female and male gametes are produced
simultaneously by a plant self fertilization can
occur.
• Offsprings produced by self fertilization are
less fit compare to outcross offsprings.
• Because of Inbreeding depression.
• Main selective forces that shape the evolution
of plant mating strategies.

• Hermaphrodite plants acquire fitness either by being
maternal or paternal parents to seeds produced in the
next generation.
• The maleness or femaleness of the plant, measured
as the proportion of a plant’s genes that are
transmitted to offspring as a pollen or ovule parent.
• Plant gender provides a functional rather than a
morphological view of plant sex.

Strategies promoting cross pollination
• Herkogamy
• Dichogamy
• Self-incompatability
• Male sterility

Sexual polymorphism
• Polymorphism:
Occurrence of two or more different morphs or
forms
• Heterostyly
• Stigma height dimorphism
• Enantoistyly
• Flexistyly

Heterostyly
Population is composed of distyly and tristyly
Distyly is most common type
Long styled called pin type
Short styled called thrum type
Heteromorphic incompatability system that prevents
selfing and intramorph mating
Controlled by supergene

Heterostyly originated in 28 diverse
animal-pollinated families
Primulaceae Linaceae Lythraceae
Turneraceae Erythroxylaceae Iridaceae

Function of heterostyly ?
“I do not think anything in my scientific life has given
me so much satisfaction as making out the meaning
of the structure of heterostylous flowers”
Darwin 1876

• Why Heterostyly will be having
reciprocal sex organ ?

Stigma height dimorphism
• Characterized by the presence of two morphs in the
population differing only in style heights.
• Some individuals present flowers with stigmas
positioned above anthers (L-morph), while others
have stigmas below anthers (S-morph)
• Boraginaceae, Linaceae, Primulaceae and Rubiaceae

Enantiostyly
Mirror image flowers in which style bends either to the
left side or right side of the floral axis.
Types
 Monomorphic enantiostyly
 Dimorphic enantiostyly
• Monomorphic enantiostyly is common in monocotyledons and
dicotyledons
• Dimorphic enantiostyly is observed in three monocotyledon
families
• Haemodoraceae, Tecophilaeaceae, Pontederiaceae
Barret, 2002

Solanum rostratum
Barret, 2002

 To investigate whether enantiostyly is
genetically determined
 controlled crosses on Heteranthera multiflora

Flexistyly
• Recently been reported from the Zingiberaceae
• It combines reciprocal herkogamy and dichogamy in a single
floral strategy
• Hyperflexistylous morph
• Cataflexistylous morph
• Flexistyly is reported from at least three clades and 24 species
in Alpinia, Amomum and Etlingera

Alpinia
Barret, 2002

MAJOR EVOLUTIONARY
TRANSITIONS
EVOLUTIONARY TRANSITIONS:

• The evolution of selfing from outcrossing
• The evolution of separate sexes
• The evolution of wind pollination from animal
pollination
EVOLUTIONARY TRANSITIONS

The evolution of selfing from
outcrossing
Why selfing evolves?
 Scarce of pollinators
 Genetic transmission advantage

Reproductive assurance
• Selfing population occupy range margins
• Ecological marginal sites with reduced
pollinator densities where outcrosses are
absent.

Automatic selection
Self pollination is mediated by pollen vectors
and the mating system modifier experiences a
transmission bias through the pollen that lead
to its selection.

How selfing is evolved
Replacement of Self incompatible
heterostyles with self compatible homostyles
with anther and stigma in close contact

Evolutionary breakdown of distyly
in Turnera
• Neotropical bee pollinated herb
Results:
Barret, 1989

Evolutionary breakdown of tristyly
in Eichhornia
• Neotropical bee pollinated herb
Barret et al. (2009)

Evolutionary pathways to self
fertilization in a tristylous plant species
Eichhornia paniculata
Barret et al . (2009)

Bakers Law
Baker’s law refers to the
tendency for species that
establish on islands by long-
distance dispersal to
show an increased capacity for
self-fertilization because of
the advantage of self-
compatibility

enrichment of self-fertility in species that established
on islands following long-distance dispersal
Single individual is sufficient to start a new
population
Pannell et al.,2015

The evolution of separate sexes
• 10% of flowering plants have unisexual
flowers.
• This condition is called Dicliny
Two categories of Gender variation is
recognized,
 Gender monomorphism
 Gender dimorphism

Sagittaria latifolia

The evolution of separate sexes
Evolutionary pathway
Selective mechanisms
Comparative biology

Male sterility
mutations
Recessive
mutation of
female able to
produce pollen
e
Sterility
muatation
Genetic modifiers
of female fertility
gradually convert
hermaphrodite to
males
Disruptive
selection
Barret, 2002

Selective mechanisms
• The theory of nuclear inheritance of male sterility indicates
that females can spread in ancestral cosexual populations if the
product of the hermaphrodite selfing rate (r) and inbreeding
depression (δ) exceeds 0.5.
• Polyploidy disrupt the functions of self incompatability,
resulting in selfing and inbreeding depression.
• In Lythium and Lycium this mechanism has been observed

• Gender dimorphism involves large plant size
• Considerable amount of selfing can occur because of the
presence of many open flowers on a plant at the same time.
• This kind of mechanism enhance Gietonogamy condition.
• Eg: Zostera marina
• Many dioecious plants are large, indicating that
geitonogamous selfing in ancestral cosexual populations might
have been important in the evolutionary origin of dioecy.

Comparative biology
• Frequency of angiosperm species that are dioecious is only 6%
• Sister-group comparisons of angiosperm taxa with contrasting
sexual systems, showed that dioecious lineages have fewer
species than their cosexual sister taxa at both the family and
genus levels.
• Dioecy is commonly associated with unspecialized pollination
systems that involve wind, water or generalist pollinators,
rather than the more specialized pollinators.

Plants with separate sexes
Theory predicts sex ratios should be 1:1
Are they?

survey resulted in sex
ratios for 243 species
representing 123
genera and 61 families
from 144 publications
between 1942 and
2010
Male bias sex ratios twice as common as female

The evolution of wind pollination
from animal pollination
• 10% of angiosperm species rely on wind pollination.
• Evolved at least 65 times from animal pollinated
ancestors.
• Wind pollination is a random and wasteful process
involving huge loss of male gamets during pollen
dispersal.

If wind pollination is so inefficient,
why it is evolved ?
It provides reproductive assurance in the
same way as self pollination relieves pollen
limitations.

• Passive process
• Pollen release, transport and deposition depend
largely on abiotic factors.
• Wind pollinated species inhabit relatively open
areas and disperse pollen in dry environment.
• Common in higher latitude and elevations
especially in temperate areas.
• Rare in tropics especially in lowland rainforest.

Reasons for evolution of wind
pollination
Complete pollinator loss
Temporal/spatial variations
in pollinator visitation
Competition among plant
species for pollinators

Trait Wind pollination Biotic pollination
Floral morphology
Stigmas Feathery Simple
Pollen ovule ratio High Low
Pollen diameter 10-50ϻm
Highly variable (often
>60ϻm)
Pollen ornamentation
Smooth with
reduced/absent pollenkitt
Often elaborate with
pollenkitt
Pollen aperture
number and type
Few, circular Numerous, elongate
Stamen filaments Long Variable
Nectaries Absent or reduced Present
Fragrance Absent or reduced Present
Perianth Absent or reduced Showy
Flower type Usually unisexual Usually bisexual
Inflorescence structure
Pendulous, catkin-like,
often condensed
Variable, sometimes
simple and diffuse
Inflorescence position Held away from vegetation Variable

Trait Wind pollination Biotic pollination
Habitat
Optimum wind speed Low to moderate Zero to low
Humidity Low Medium to high
Precipitation Infrequent
Infrequent to
common
Surrounding
vegetation
Open Open to closed
Plant density Moderate to high Low to high

T. occidentale T. uchiyamai

• The evolution of wind pollination requires not
only wind
• It requires low humidity, low precipitation,
open vegetation and intercompatible
gregarious plants
• Ambophily

Hermaphroditism animal
pollination outbreeding
Loss of SI altered
floral traits
Spread of sterility
mutations
Loss of adaptations
for animal
pollination
Selfing Dioecy Wind pollination

Bird-pollinated species of Babiana (Iridaceae) Rat’s tail
Enigmatic species
endemic to the
Western Cape
Province, South
Africa
Ground flowering
and naked
inflorescence axis

What
pollinates
this plant and
what is the
function of
the rat’s tail?

Primary feeding strategies of flower-visiting birds
New World Old World

A novel example of a structural adaptation that
promotes cross pollination in angiosperms
Bruce Anderson, William W. Cole, Spencer C. H.
Barrett, 2005

They investigated the curious sterile inflorescence axis
of the ‘rat’s tail’ plant
Sterile inflorescence axis-provide a perch for foraging
birds
malachite sunbird (Nectarinia famosa), its main
pollinator

Cape naturalist Rudolf
Marloth was the first to
propose that the rat’s tail of
B. ringens could function as
a perch to facilitate cross-
pollination by visiting
sunbirds
They investigated this
proposal in two populations
of B. ringens near Mamre

 Only pollinator was the
malachite sunbird birds
alighting on the sterile axis
of the plant and rotating
upside down

Testing the function of the bird perch
 Perch removal did not alter the floral display
or the intrinsic ability of plants to set seed
 They found no significant difference in seed
set between plants with and without perches

A fraction of each of two plant populations (A and B, represented by filled and open circles)

Take home messages
Natural history observations, field experiments
and genomics can provide novel insights into
plant sex
Many fascinating floral adaptations still to be
discovered
Plant sex never stops evolving and is worth a
look – to become a ‘Plant sex voyeur’

T
Thank you
Diversity jolts us into
congitive action in ways that
homogenity simply does not

Evoluton of plant sexual diversity

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