Transcript Plant breeding systems

Genetics of
Plant Breeding Systems
Promoting Outcrossing
Review
• no direct relation between DNA change and
functional (“phenotypic”) change
• ratio of nonsynonymous to synonymous mutations
within and among species indicates intensity of
selection
• gene inactivation, regulatory evolution through
cis-acting elements are important evolutionary
forces leading to new morphological forms
Review
• origin of new genes through polyploidy,
duplications, imported DNA
• comparisons of proteomes indicates range of
change (single substitution leading to dramatic
change, or conservation of function with extensive
amino acid replacement)
• comparisons of genomes shows conservation of
gene order
Review
• major theoretical model of speciation is
allopatric, with initial geographic separation
• prezygotic and/or postzygotic isolation
gradually lead to genetic and morphological
differentiation
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