Transcript Plant breeding systems

Genetics of
Plant Breeding Systems
Promoting Outcrossing
Angiosperm breeding systems
Plants have creative ways to reproduce successfully—extremes from
obligate selfing to obligate outcrossing
Breeding systems enforcing
outcrossing
• evolutionarily advantageous (in theory) to prevent
pollination between closely related individuals
• major mechanisms enforcing outcrossing (crosspollination)
– self-incompatibility—negative chemical interaction
between pollen and style tissue with same alleles
– heterostyly—mechanical prevention of pollen
deposition by relative placement of anthers to style
– dioecy—separation of anthers and pistils on separate
plants
Self-incompatibility systems in
angiosperms
• evolutionarily advantageous to enforce
“outcrossing”—pollination among unrelated
individuals
• self-incompatibility (SI) mechanism one way to
accomplish this, by blocking selfing or sib mating
• self-incompatibility (SI) well studied in some
plants, based on protein-protein interactions
between pollen and style involving S-locus genes
Self-incompatibility systems in
angiosperms
• S-locus genes have many different alleles in
a given population
• interaction of proteins on pollen and style
with same alleleSI response (no pollen
tube growth)
• interaction between pollen and style with
different allelesno SI response (successful
fertilization)
Self-incompatibility systems in
angiosperms
• different plant families have evolved one or the
other of 2 mechanisms (plus a smattering of
others)
• but many plants are self-compatible (estimated
50% of angiosperms)
• 2 major SI mechanisms:
– gametophytic SI—pollen phenotype is determined by
its gametophytic haploid genotype
– sporophytic SI—pollen phenotype is determined by
diploid genotype of the anther
Sporophytic SI mechanism
• in sporophytic SI, S-locus is cluster of three
tightly-linked loci:
– SLG (S-Locus Glycoprotein)—encodes part of
receptor present in the cell wall of the stigma
– SRK (S-Receptor Kinase)—encodes other part
of the receptor
– SCR (S-locus Cysteine-Rich protein)—encodes
soluble ligand for same receptor
Sporophytic SI mechanism
• in sporophytic SI, S-locus is
cluster of three tightly-linked loci:
– SLG (S-Locus Glycoprotein)—
encodes part of receptor present in
the cell wall of the stigma
– SRK (S-Receptor Kinase)—encodes
other part of the receptor.
– SCR (S-locus Cysteine-Rich
protein)—encodes soluble ligand for
same receptor
• only pollen grains from
heterozygote for S-alleles will
germinate
Gametophytic SI mechanism
• more common than
sporophytic SI but less well
understood
• SI controlled by single S
allele in the haploid pollen
grain
• only pollen grains not
containing same allele as
style tissue will germinate
S1 S2
S1 S2
S1S2 pistil
S1 S2
S1S3 pistil
S3S4 pistil
Evolution of self-incompatibility:
S-locus in Maloideae
• Raspé and Kohn (2007)
genotyped stylarincompatibility RNase in
20 pops of European
mountain ash (Sorbus
aucuparia)
• found up to 20 different
alleles in some pops
• recovered total of 80 Salleles across populations-huge diversity
Self-compatibility in Arabidopsis
thaliana
• Broyles et al. (2007)
discovered that loss of selfincompatibility (ancestral
condition) in Arabidopsis is
associated with inactivation of
genes required for S1—SRK
and SCR
• divergent organization and
sequence of
haplotypesextensive
remodeling, reversal of selfincompatibility
S-allele diversity and real-life
populations: the pale coneflower
S-allele diversity and real-life
populations: purple coneflower
• Wagenius et al. (2007) examined seed set in selfincompatible purple coneflower in various-sized
prairie fragments
• pollination and new seeds increased with pop
density—”Allee effect” based on increased
diversity of S-alleles
• simulation modeling: small pop sizeslowered
seed set due to loss of S-alleles through drift
Heterostyly as another outcrossing
mechanism
• described in detail first by
Darwin, in purple loosestrife
(Lythrum salicaria)
• different individuals have
floral forms differing in
relative positions of stigma
and anthers (distyly—2
forms, tristyly—3 forms)
• pollination effective only
between different floral
forms on different
individuals
Heterostyly as another outcrossing
mechanism
• both heterostyly and any associated incompatibility
reactions controlled by "supergenes“
• in distyly, thrum plants are heterozygous (GPA/gpa)
while pin plants are homozygous (gpa/gpa):
– female characters controlled by G supergene—G = short
style, g = long style
– male characters controlled by P supergene—P = large pollen
& thrum male incompatibility, p = small pollen & pin male
incompatibility
– anther position controlled by A supergene—A = high anthers
(thrum), a = low anthers (pin)
Heterostyly and polyploidy in
primroses
• Guggisberg et al. (2006)
analysed phylogenetic
relationships of a
primrose group using 5
chloroplast spacer genes
• interpreted 4 switches
from heterostyly to
homostyly and 5
polyploid events
• all homostyly switches
correspond to polyploidy
red depicts homostylous species
Heterostyly and polyploidy in
primroses
• all homostyly switches correlate precisely with
polyploid events
• polyploids inhabit more northerly regions left
vacant by retreating glaciers in last 10,000 years
• outcrossing in those regions may not have been as
important for reproductive success as selfing,
according to surmise of authors
• additional idea—does polyploidy modify genetics
of heterostyly?
Dioecy as a third outcrossing
mechanism
• dioecy—individuals possessing either
stamens or carpels (separation of sexes on
different plants)
• frequent in temperate trees, annual weeds,
few forest herbs, especially common in
oceanic island archipelagos
• totals ca. 4% of angiosperms
Dioecy as a third outcrossing
mechanism
• frequent in temperate trees and annual
weeds, especially common in oceanic island
archipelagos
• another successful strategy for ensuring
cross-pollination among unrelated plants
Typical developmental basis of dioecy
• buds originate as normal bisexual flowers,
with anther and pistil meristems
• at some point in early flower development,
further elaboration is halted in one or other
reproductive structure
• flower becomes functionally staminate or
pistillate (many species retain vestigial
parts, showing basis of unisexual flowers)
Dioecy and monoecy
interconvertible
• Zhang et al. (2006)
examined Cucurbitales
order (including
begonias, gourds)
using 9 chloroplast
genes
• found repeated
switches between
bisexuality, monoecy
and dioecy—very
labile
Molecular basis of dioecy in Thalictrum
• di Stilio (2006) studied
molecular correlates of
development in meadow rue
(Thalictrum), a windpollinated dioecious forest
herb
• found that earliest flower
buds were already either
carpellate or staminate—
suggested homeotic gene
regulation
carpellate
staminate
bisexual
relative
Floral homeotic (ABC) genes
• well known model describes
floral organ identity by
major classes of genes
• various homologs of each
class have been identified in
different plants studied,
A
including:
– apetala3 (AP3), B class
– pistillata (PI), B class
– agamous (AG), C class
B
C
sepals
petals stamens carpels
Floral homeotic (ABC) genes
• in other groups, mutations in
B class genes in other plants
produce carpellate flowers
• overexpression of B class
genes produces staminate
flowers
A
• hypothesis of di Stilio et al.:
sexual dimorphism of dioecy
based on differential
regulation of B and C genes
B
C
sepals
petals stamens carpels
Returning now to our Thalictrum
program...
• investigators recovered
several AP3 homologs
(left tree) and 2 PI
homologs (right tree)
• 3 AG homologs also
found
• AP3 homolog sequences
are truncated with a
premature stop
codonno effective
protein produced
Returning now to our Thalictrum
program...
• RT-PCR with locus-specific
primers in dioecious species
used
• showed expected gene
expression pattern:
staminate flowers have B
class AP3 and PI homologs
and AG1 homolog expressed
carpellate flowers have
only AG2 (carpel-specific)
homolog expressed
Summary
• plant breeding systems span range from
obligately selfing to obligately outcrossing
• various strategies have evolved to promote
outcrossing; major ones are:
– self-incompatibility—chemical control of
pollen germination on style
– heterostyly—mechanical prevention of pollen
deposition by relative displacement of anthers
and stigma
Summary
– dioecy—separation of sexes on different plants
• each breeding system has different
molecular genetic regulation
• breeding systems can flip-flop back and
forth, even within lineages—evolutionarily
labile