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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
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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 (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 S3S4 pistil S1S3 pistil S1S2 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 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
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 B C A petals sepals carpels stamens
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 B C A petals sepals carpels stamens
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 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