Polyploidy

I. Types of polyploidyA. Autopolyploidy: more than 2 genetically identical genomes

B. Allopolyploidy: combines the genomes of more than one species

C. Intermediate situations, e.g. segmental allopolyploids

D. Ancient polyploidy followed by chromosomal repatterning and restoration of diploid-like chromosome behavior "diploidization"

II. Frequency of Polyploidy

A. 30-35% (Stebbins) - 70-80% (Lewis) of angiospermsdepends if only "multiples" within a genus are counted or if n > 10 is counted

B. Support for higher estimate

1. Smaller stomata in Miocene fossils supports higher estimate

2. Evidence for genome duplication Arabidopsis n = 5

C. Varies across taxonomic groupse.g. Common in ferns [20% in North America] but not conifers

Duplicated sections of the Arabidopsis genome

III. Genetics of Polyploids

A. Allopolyploidy & Isozymes

1. fixed heterozygosity

2. disomic inheritance

AAaa X AAaa - > AAaa

B. Autopolyploidy & Isozymes

1. subset of progenitor’s alleles

2. tetrasomic inheritance

AAaa X AAaa - > 1 AAAA8 AAAa18 AAaa8 Aaaa1 aaaa

e.g. Tolmiea menziesii

diploid

diploid

SkDH PGM

PGM TPI

TPI PGI

Tolmiea menziesii

IV. Multiple origins of polyploids

A. Different fixed heterozygote genotypes in allotetraploids

B. Different cpDNA haplotypes

C. Different ITS repeats through concerted evolution

D. Documented in Tragopogon, Brassica triangle, various ferns

E. results in polyphyletic species, if independent polyploids are interfertile

2n

4n

2n

4n

2n

4n

Hybridization and Introgression

hybridization = F1

“crosses between genetically differentiated taxa”

introgression = F2 backcrosses

“movement of genes between species (or other well-marked genetic populations) mediated by back-crossing”

I. Hybrid zones

A. Observed in many species

Longstanding questions:

1. primary speciation or secondary intergradation?

2. Source of new species or just "evolutionary noise" ?

B. Ecology of hybrid zones

1. Often in intermediate habitats

2. Often in disturbed habitats

L. leucophyllus × sericeus

L. sericeus var. egglestonianusL. leucophyllus L. sericeus

L. latifolius var. latifolius × sericeus = L. latifolius var. thompsonianus

3. Conservation concerns over loss of endemics due to hybridization with a widespread taxon

e.g. Norway spruce swamping Serbian spruce

Penstemon parryi used in post-fire restoration & highway planting

II. Diploid Hybrid Speciation

considered uncommon, < 10 documented casesbest known in Helianthus

A. fertile F1

B. Stabilization of hybrid segregants

C. How does it become reproductively isolated?

C. How does it become reproductively isolated?

1. postmating: recombinational speciation via

chromosomal rearrangements

2. premating: ecological divergence e.g. habitat or pollinator

D. Polarity assessment can be problematic, because ploidy level cannot be used as a clue to parentage

III. Introgression

"genetic exchange between hybridizing species"

A. Asymmetry of gene flow

1. Nuclear genes transferred, cpDNA note.g . Long-distance pollen movement

2. Chloroplast capture: cpDNA transferred, not nuclear genes

3. potential for cytoplasmic incompatibility and reproductive isolation(Levin, Syst. Bot. 28:5-11, 2003)

Morgan, D.R. 2003. Systematic Botany 28: 179–190. nrDNA External Transcribed Spacer (ETS) Sequence Data, Reticulate Evolution, and the Systematics of Machaeranthera (Asteraceae)

homoploid reticulation= introgression

Machaeranthera gracilis2n = 4

M. tanacetifolia

J. SempleJ. Semple