Genetics ofPlant Breeding Systems Promoting Outcrossing

Angiosperm breeding systems

Plants have creative ways to reproduce successfully—extremes fromobligate selfing to obligate outcrossing

Breeding systems enforcing outcrossing

• evolutionarily advantageous (in theory) to prevent pollination between closely related individuals

• major mechanisms enforcing outcrossing (cross-pollination)– 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 alleleSI response (no pollen tube growth)

• interaction between pollen and style with different allelesno 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 S1 S2

S1S2 pistil S1S3 pistil S3S4 pistil

Evolution of self-incompatibility:S-locus in Maloideae

• Raspé and Kohn (2007) genotyped stylar-incompatibility RNase in 20 pops of European mountain ash (Sorbus aucuparia)

• found up to 20 different alleles in some pops

• recovered total of 80 S-alleles across populations--huge diversity

Self-compatibility in Arabidopsis thaliana

• Broyles et al. (2007) discovered that loss of self-incompatibility (ancestral condition) in Arabidopsis is associated with inactivation of genes required for S1—SRK and SCR

• divergent organization and sequence of haplotypesextensive remodeling, reversal of self-incompatibility

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 self-incompatible 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 sizeslowered 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 wind-pollinated dioecious forest herb

• found that earliest flower buds were already either carpellate or staminate—suggested homeotic gene regulation

carpellate

staminate

bisexualrelative

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, including:– apetala3 (AP3), B class

– pistillata (PI), B class

– agamous (AG), C class

sepals petals stamens carpels

A C

B

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

• hypothesis of di Stilio et al.: sexual dimorphism of dioecy based on differential regulation of B and C genes

sepals petals stamens carpels

A C

B

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 codonno 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 expressedcarpellate 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