Plant Breeding Systems Promoting Outcrossing 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 (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 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: 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

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

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

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

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!

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

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

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

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

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 nearly all homostyly switches correspond to polyploidy red depicts homostylous species

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

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)

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

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

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

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

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