Reading - The role of hybridization in plant speciation Flashcards
Hybrid speciation at:
- polyploid level
- homoploid level
- autopolyploid level
Polyploidy
more than 2 sets of homologous chromsomes
Homoploid level
betewen 2 species of the same ploidy
(same number of homologous chromosomes)
Autopolyploidy
genome doubling within species
(or between populations of the same species)
Hybridization
crossing betwee:
- species
- genetically divergent populations
- races wihtin a species
Speciation in plants and animals:
similarities
- gene flow in plants is more than originally thought
- species reproductively isolated
Speciation in plants and animals:
differences
- the high frequency of hybridization and its role in speciation
- significant proportion of speciation in plants via hybridization → phylogenetic net, not bifurcating tree
Allopolyploid speciation is more common than homoploid speciation
- homoploids have reduced fitness in early generation hybrids
- not in allopolyploids - though possibile sterility bottleneck
- genome doubling in allopolyploids
- no/less backcrossing with parents
- not the case in homoploids
→ new species via allopolyploidy are more likely
Many/most angiosperms are of
ancient polyploid origin
hybridization and genome doubling → species diversity
Allopolyploidy is more common no, so extrapolate that
most ancient polyploid events were allopolyploid
Masterson
- studied leaf guard cells in fossils/relatives
- found 70% of angiosperms have experienced polyploidy
- by complete sequencing of the nuclear genome
- found ancient polyploidy in angiosperms and eukaryotes
- by sequencing all plant genomes
- fond ancestor of Arabidopsis = hexaploid, subsequent duplications of Brassicales to get to Arabidopsis
Lynch and Conery
used genomic datat to infer occurrences of ancient genome duplication
- used expressed sequence tags
- to see if polyploid event occurred and when
- found ancient polyploidy in a number of crops with multiple genome duplicationsi n some
Amborella
sister to angiosperms
- shows no evidence of ancient polyploidy
- signature may just be erased
Recept (within 150 years) polyploids
- S. anglica (grass)
- T. mirus and T. miscellus (flowers)
S. anglica shows
few changes in the genome but changes in methylaiton and epigenetic programming
Recent allotetraploids
- Senecio and Tragopogon
- loss of homeologues and DNA sequences, change in DNA expression
Types of polyploids
- autopolyploids - multiplication of 1 chromosome set
- allopolyploids - from merger of structurally different chromosome sets
- segmental allopolyploids (Stebbins) - polyploids that comprise slightly differentiated chromosome sets
Allopolyploids form hybrids between
distantly related species
→ combine divergent genomes with chromosomal complements that can’t pair
Autopolyploid formation
- doubling within an individual or crossing bewtween different populations within a species
- with production and merger of unreduced gametes from genetically and chromosomally similar individuals
- genetically different ~= speciation
Allpolyploids/allotetraploids - expect
additivity of parental genotypes
BUT - will sequester parental genetic variation into its component genomes
→ some genetic diversity will segregate, some won’t
genetic variation in homologous chromosomes will segregate
genetic variation in homeologous chromosomes won’t
Parents fixed for alternative alleles at all loci
→
complete additivity of the parental genes, appear heterozygous at all homeologous loci
- segregating allelic variation between homologues
- nonsegregating fixed heterozygosity at all homeologous loci
Nearly all polyploid species comprise populations of independent formation from genetically distinct progenitor populations
- if each of several constituent populations of each allotetraploid species of multiple origins had both segregating variation and fixed heterozygosity, then each allotetraploid populaiton would have its own set of genotypes and all populations would be genetically distinct
- novel genotype from crossing between genetically different individuals
Polyploid evolution
- genomes merge → genomic shock
- newly formed polyploid genomes undergo movement of transposable elements and rapid changes in the genome size and structure, epigenetic control
- polyploid → restructuring of transcriptome, metabolome, proteome
Brassica
- structural changes in first few generations
Tragopogon
- major structural changes (including translocations) in natural populations not more than 30-40 generations
Polyploidy → changes in:
- gene silencing
- DNA methylation
- tissue-specific expression
= role in modifying patterns of gene expression
→ source of genetic novelty to play a role in evolutionary success
- individuals with modified phenotype, ecological preferences
- to exploit new niches and outcompete competitors
Are rates of diversification higher in diploids or polyploids?
Soltis
- compared species richness in clades that are ancient polyploids with sister clades that aren’t
→ polyploidy = major force behind diversification of angiosperms
→ genome doubling may have led to a dramatic increase in species richness ins esveral angiosperm lineages
Conditions that favor polyploidization v hybridization
- closely related diploids are less likely to form a polyploid than are more divergent diploid species
- more distantly related diploids make polyploids
- successful allopolyploids could be derived more easiliy than homoploids from distantly related parents
- distant homoploids are more successful allopolyploids than close (Digby)
- parents of allopolyploids typically twice as divergent as parents of homoploid hybrid species (Chapman)
homoploids - parents of low divergence
allopolyploids - parents of high divergence
Muntzing
- chromosomal races within species are distinct evolutionary lineages
- 58 chromsome races
- autopolyploid morphologically distinct from diploid parent, chromosomal races reproductively isolated
Allopolyploids are more common than
autopolyploids
- autopolyploids difficult to detect)
Methods of polyploidization
- fusion of unreduced gametes more likely than somatic doubling
- unclear how frequent a 1-step (fusion of unreduced gametes) is vs. 2-step via triploid bridges
- 1 haploid gamete + 1 unreduced gamete
- then fusion of triploid gamete with reduced gamete → tetraploid
- recurrent formation from genetically distinct diploid parental populations introduces genetic variation into a polyploid species
Werth and Windham
- if alternative homologs were silenced across an allopolyploid genome in different polyploid individuals (reciprocal silencing)
→ reproductive isolation and incipient speciation
Polyploid populations of independent origin
(formation from genetically divergent parents)
may have distinct and cytogenetic signatures
- changes in chromosomes might be responsible for reproductive isolation between allopolyploid populations
(Ownbrey and McCollum)
Ownbrey and McCollum
- crossed reciprocally formed populations of T. miscellus
- reproductive barriers between populations (chromosomal changes)
→ if different population s have different chromosomal attributes there may be reduced fertility between, or reproductive isolation
Preservation of duplicated gene copies
nonrandom
- some functional categories preferentially retained, reduplicated
see Gaeta
Gaeta
- using identically produced synthetic lines of B. napus, showed repeatability of the evolutionary process
- changes in expression, homeologue loss, structural changes occurred across 50 separate polyploid lines
- → functional categories preferentially retained, reduplicated (preservation of duplicated gene copies)
Tragopogon populations of independent origin
show evidence of repeated gene loss
- preservation of duplicated gene copies
- some functional categories preferentially retained, reduplicated
Even if a hybrid survives and reproduces
subsequent generations may be increasing less fit
= hybrid breakdown
- may not have a sustainable niche, forced into parental where less fitlow frequency of mate → excluded from population with parents
- repeat exclusion → maintain integrity of parental species, long-term barrier to interspecific mating
Introduced species
- other pollinators visit close relatives → hybridize
- Tragopogon in N. america plus pollinators that don’t discriminate
- change in phenology → overlap flowering times → hybridization
- climate change → change in distibution and phenology
