1. Plant Breeding Systems Promoting Outcrossing
Genetics of
Plant Breeding Systems Promoting Outcrossing

2. Angiosperm breeding systems
Plants have creative ways to reproduce successfully—extremes from
obligate selfing to obligate outcrossing

3. 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

4. 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
SI well studied in some plants, based on protein-protein interactions between pollen and style involving S-locus genes

5. 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)

6. 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:
sporophytic SI—pollen phenotype is determined by diploid genotype of the anther tapetum on the grain
gametophytic SI—pollen phenotype is determined by gametophytic haploid genotype inside the pollen grain

7. 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
Remnants of diploid tapetum on pollen grains harbor S-alleles from staminate plant
If genotypes of staminate plant different from pistillate plant, no SI response  pollen tubes will germinate and grow normally

8. Gametophytic SI mechanism
more common than sporophytic SI but less well understood
SI controlled by single S allele in the haploid pollen grain; here, the pollen grain’s genotype itself governs the SI response
If pollen grain genotype differs from either allele in pistillate plant, no SI response  pollen tubes will germinate and grow
S1 S2
S1 S2
S1 S2
S1S2 pistil
S1S3 pistil
S3S4 pistil

9. Evolution of self-incompatibility: S-locus in Maloideae
European mountain ash
(Sorbus aucuparia)

10. 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 of alleles!

11. Self-compatibility in Arabidopsis thaliana
Arabidopsis relative
with SI gene complexes
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 haplotypesextensive remodeling, reversal (=loss) of self-incompatibility
Arabidopsis thaliana

12. S-allele diversity and real-life populations: the pale coneflower
Pale coneflower (Echinacea pallida)

13. S-allele diversity and real-life populations: the pale coneflower

14. 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 sizeslowered seed set due to loss of S-alleles through drift

15. Heterostyly as another outcrossing mechanism
Purple loosestrife (Lythrum salicaria)
Primrose
(Primula sp.)

16. 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

17. 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)

18. 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
nearly all homostyly switches correspond to polyploidy
red depicts homostylous species

19. Heterostyly and polyploidy in primroses
nearly all homostyly switches correlate 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
unclear exactly how does polyploidy modifies genetics of heterostyly

20. Dioecy as a third outcrossing mechanism
bisexuality—individuals possessing stamens and pistils in same flower; most common reproductive mode
monoecy—individuals possessing unisexual (staminate or pistillate) flowers in separate areas
dioecy—individuals possessing either stamens or carpels (separation of sexes on different plants)
frequent in temperate trees, annual weeds, few forest herbs
totals ca. 4% of angiosperms worldwide
especially common in oceanic island archipelagos; 14.7% of angiosperms in Hawaii are dioecious (Sakai et al. 1995)

21. 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)

22. 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

23. Molecular basis of dioecy in Thalictrum
carpellate
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
staminate
bisexual
relative

24. 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), A class
pistillata (PI), B class
agamous (AG), C class
pistillata B A C
sepals
petals
stamens
carpels
apetala3
agamous

25. 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
pistillata B A C
sepals
petals
stamens
carpels
agamous

26. 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under-developed sepals, petals
AP3 PI

27. 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

28. 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

29. each breeding system has different molecular genetic regulation
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