Reading - The role of hybridization in plant speciation

Question 1

Hybrid speciation at:

Answer 1

• polyploid level
• homoploid level
• autopolyploid level

Question 2

Polyploidy

Answer 2

more than 2 sets of homologous chromsomes

Question 3

Homoploid level

Answer 3

betewen 2 species of the same ploidy

(same number of homologous chromosomes)

Question 4

Autopolyploidy

Answer 4

genome doubling within species

(or between populations of the same species)

Question 5

Hybridization

Answer 5

crossing betwee:

• species
• genetically divergent populations
• races wihtin a species

Question 6

Speciation in plants and animals:

similarities

Answer 6

• gene flow in plants is more than originally thought
• species reproductively isolated

Question 7

Speciation in plants and animals:

differences

Answer 7

• the high frequency of hybridization and its role in speciation
  
  • significant proportion of speciation in plants via hybridization → phylogenetic net, not bifurcating tree

Question 8

Allopolyploid speciation is more common than homoploid speciation

Answer 8

• 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

Question 9

Many/most angiosperms are of

Answer 9

ancient polyploid origin

hybridization and genome doubling → species diversity

Question 10

Allopolyploidy is more common no, so extrapolate that

Answer 10

most ancient polyploid events were allopolyploid

Question 11

Masterson

Answer 11

• 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

Question 12

Lynch and Conery

Answer 12

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

Question 13

Amborella

Answer 13

sister to angiosperms

• shows no evidence of ancient polyploidy
  
  • signature may just be erased

Question 14

Recept (within 150 years) polyploids

Answer 14

• S. anglica (grass)
• T. mirus and T. miscellus (flowers)

Question 15

S. anglica shows

Answer 15

few changes in the genome but changes in methylaiton and epigenetic programming

Question 16

Recent allotetraploids

Answer 16

• Senecio and Tragopogon
• loss of homeologues and DNA sequences, change in DNA expression

Question 17

Types of polyploids

Answer 17

• autopolyploids - multiplication of 1 chromosome set
• allopolyploids - from merger of structurally different chromosome sets
• segmental allopolyploids (Stebbins) - polyploids that comprise slightly differentiated chromosome sets

Question 18

Allopolyploids form hybrids between

Answer 18

distantly related species

→ combine divergent genomes with chromosomal complements that can’t pair

Question 19

Autopolyploid formation

Answer 19

• 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

Question 20

Allpolyploids/allotetraploids - expect

Answer 20

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

Question 21

Parents fixed for alternative alleles at all loci

→

Answer 21

complete additivity of the parental genes, appear heterozygous at all homeologous loci

• segregating allelic variation between homologues
• nonsegregating fixed heterozygosity at all homeologous loci

Question 22

Nearly all polyploid species comprise populations of independent formation from genetically distinct progenitor populations

Answer 22

• 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

Question 23

Polyploid evolution

Answer 23

• 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

Question 24

Brassica

Answer 24

• structural changes in first few generations

Question 25

Tragopogon

Answer 25

* major structural changes (including translocations) in natural populations not more than 30-40 generations

Question 26

Polyploidy → changes in:

Answer 26

* 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

Question 27

Are rates of diversification higher in diploids or polyploids? Soltis

Answer 27

* 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

Question 28

Conditions that favor polyploidization v hybridization

Answer 28

* 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

Question 29

Muntzing

Answer 29

* chromosomal races within species are distinct evolutionary lineages * 58 chromsome races * **autopolyploid morphologically distinct from diploid parent, chromosomal races reproductively isolated**

Question 30

Allopolyploids are more common than

Answer 30

autopolyploids * autopolyploids difficult to detect)

Question 31

Methods of polyploidization

Answer 31

* 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

Question 32

Werth and Windham

Answer 32

* if alternative homologs were silenced across an allopolyploid genome in different polyploid individuals (reciprocal silencing) → reproductive isolation and incipient speciation

Question 33

Polyploid populations of independent origin (formation from genetically divergent parents)

Answer 33

may have distinct and cytogenetic signatures * changes in chromosomes might be responsible for reproductive isolation between allopolyploid populations (Ownbrey and McCollum)

Question 34

Ownbrey and McCollum

Answer 34

* 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

Question 35

Preservation of duplicated gene copies

Answer 35

nonrandom * some functional categories preferentially retained, reduplicated see Gaeta

Question 36

Gaeta

Answer 36

* 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)

Question 37

Tragopogon populations of independent origin

Answer 37

show evidence of repeated gene loss * preservation of duplicated gene copies * some functional categories preferentially retained, reduplicated

Question 38

Even if a hybrid survives and reproduces

Answer 38

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

Question 39

Introduced species

Answer 39

* 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