Plant Speciation

1
Plant Speciation Evolution (PBIO 475/575)

• Sources of Genetic Variation

2
Mutations

• Major categories of phenotypic effects
• Morphological
• Lethal
• Conditional
• Biochemical
• Resistant
• Frequency
• Relatively rare per gamete or per generation in
  an individual but calculating production across
  populations and species, over life spans, becomes
  significant
• e.g., ca. 1 in 10,000 for many human chromosomal
  diseases ca. 1 in 20,000 for some corn mutations

3
Mutations

• Directionality and effects
• Forward mutations by far the most common
• Reverse very very rare difficult to discern from
  forward mutations representing different
  "pathway" but restoring original phenotype
• Most mutations are deleterious
• Non-deleterious mutations usually produce minor
  phenotypic changes
• e.g., albino flowers through "turning-off" of
  pigment pathway
• e.g., hairless leaves through cessation of
  trichome development
• BUT accumulation of slight changes--major
  changes, new syndromes

4
Mutations

• Mutagenic agents
• UV light
• Radiation
• Very low and very high temperatures
• Chemicals (substrates like serpentine?)
• Altitude (related to UV increase?)
• Age of organs
• ?Increased rates of mutation, polyploidy at high
  elevations, high latitudes near poles, odd soil
  or rock substrates

5
Genetic Effects of Mutations

• Substitution of "synonymous" codon ? no net
  effect ("silent")
• Non-synonymous substitution translates to
  different amino acid

6
Point Mutations

• Locations in DNA
• Promoter
• Initiation site
• Coding region
• Analogous locations in RNA
• Mismatch of codon and anticodon ? incorrect amino
  acid substituted

Suzuki et al. (1989)
7
Point Mutations

• Mutational "hotspotsdifferent regions of a gene
  have different probabilities of change

Suzuki et al. (1989)
8
Genetic Effects of Point Mutations

• Non-synonymous substitutions (cont.)
• May affect architecture of proteins
• If protein not severely changed, has weak or no
  impact (may accumulate in populations)
• If drastically changed, has profound impacts (may
  be eliminated from population quickly)

Raven et al. (1992) Suzuki et al. (1989)
9
Genetic Effects of Point Mutations

• Frameshift mutations of regulatory or structural
  genes
• Gene doesn't end in stop codon before completion
  of translation--weak or severe impact
• Premature stop codon--mostly severe impact
• With or without subsequent "restoring" mutation
• Evolutionary consequence of non-lethal point
  mutations heritable genetic variation

10
Larger Intragenic Changes

• Crossing-over within genes
• Gene conversion
• Transposable elements
• Small segments of DNA distributed randomly
  throughout the genome
• Capable of excising and reinserting anywhere

Alberts et al. (1989)
11
Larger Intragenic Changes

• Transposable elements (cont.)
• May excise perfectly, imperfectly or duplicate
  and leave a copy behind
• Results in gene inactivation, chromosome breakage
  and rearrangements, increase in deletions around
  flanking regions of transposons, hybrid
  dysgenesis
• e.g., kernel pigmentation in Zea
• Frequency across plants still unclear (probably
  grossly underestimated)

12
Changes to Chromosome Structure

• Deletion
• Often lethal if deletion is extensive
• If not, may give homozygous recessive alleles
  "pseudodominant" expression
• Deletion and duplication events recognizable as
  "loops" during meiosis

Suzuki et al. (1989)
13
Changes to Chromosome Structure

• Duplication
• Important evolutionarily, not generally lethal
• Provides duplicate genetic material available for
  divergent functions
• Dosage effects in expression
• Duplicated segment may be tandem or reverse in
  orientation

14
Changes to Chromosome Structure

• Inversion
• Paracentric
• Centromere not included in inversion
• Crossing-over in heterozygote gives 2 homologs
  connected by dicentric bridge breaks into 2
  deletion products 2 normal, plus acentric
  fragment (later lost)

Suzuki et al. (1989)
15
Changes to Chromosome Structure

• Inversion (cont.)
• Pericentric
• Includes centromere, yields products differing in
  centromere placement
• Results in 4 normal-sized chromosomes--1 normal,
  1 inversion product, 2 duplication/deletion
  products
• Recombination in inversion heterozygotes reduced
  in 2 ways
• Inhibits crossing-over near the inversion
• Eliminates products of crossing-over in inversion
  loop (deletions)

16
Changes to Chromosome Structure

• Translocation
• Reciprocal is common, with pieces swapped between
  2 chromosomes
• May drastically change size of resulting
  chromosomes
• Cytologically detected as "cross"-shaped pairing
  configuration

Suzuki et al. (1989)
17
Changes to Chromosome Structure

• Translocation (cont.)
• Adjacent-1 or Adjacent-2 segregation
• Normal chromosome goes with translocated one in
  each case
• Often produces inviable meiotic products given
  deletions
• Alternate segregation
• Translocated chromosomes go together, normal go
  together
• Complete genetic complements--viable products in
  each case
• Gives appearance of linkage when deletion
  products are eliminated

18
Changes to Chromosome Structure

• Unequal crossing-over
• Asymmetrical pairing
• Both result in duplication and deletion products

Suzuki et al. (1989)
19
Changes in Chromosome Number

• Euploidywhole genome duplication
• Two origins
• Somatic doubling in stem--tetraploid flowers
  yield diploid gametes
• Diploid flowers produce unreduced diploid gametes
  via nondisjunction
• Types (form a continuum of sorts)
• Autopolyploidsduplication of one resident
  genome recent studies suggest it is
  comparatively rare
• Allopolyploidsduplication of hybrid genome more
  common than previously thought (covered later in
  Speciation Models and Pathways)

20
Changes in Chromosome Number

• Eupolyploids found abundantly throughout
  angiosperms and many other land plant groups, not
  so common in animals
• Many primitive groups with high base numbers (low
  ploidy levels extinct)
• "Diploidization"--homogenization of genome in
  some older groups
• Silencing of multiple copies often found in
  higher polyploids, given sufficient time from
  polyploid event

21
Changes in Chromosome Number

• Aneuploidschanges in number of individual
  chromosomes
• Origin by nondisjunction
• At first division--2 n1 and 2 n-1 gametes
• At second division--1 n1, 1 n-1, 2 normal
  gametes
• Monosomics (2n-1) and nullisomics (2n-2)
• Usually deleterious or lethal given meiotic
  imbalance in diploids
• In polyploids like wheat, imbalance is
  "tolerated"
• Trisomics (2n1)
• More often viable
• Sometimes fertile

22
Somatic Mutation Revisited

• Genetic Mosaicism Hypothesis (Whitham
  Slobodchikoff 1981 Gill 1986)
• Cells in tissues might accumulate innumerable
  small somatic mutations independently
• Genetic heterogeneity within individuals provide
  adaptive advantages in pest resistance, herbivore
  defense
• If mutations occur in tissues that will produce
  gametes, somatic mutations will be heritable
• Would be most easily observed in long-lived or
  extensively clonal organisms

23
Somatic Mutation Revisited

• Limited empirical genetic support--due to
  difficulty of assessing variation at low levels
  but lots of morphological evidence becoming
  widely accepted as a plausible theory
• May be an overlooked and potentially important
  source of heritable variation
• e.g., where mutations have occurred in tissues
  that will produce sexual organs (lateral branch
  primordia that will generate inflorescences and
  flowers)
• possible basis for polyploidy?somatic doubling
  in lateral branches, later yielding flowers with
  2X gametes

24
Bibliography

• Alberts, B., D. Bray, J. Lewis, M. Raff, K.
  Roberts, and J. D. Watson. 1989. Molecular
  biology of the cell, 2nd ed. Garland Publishing,
  Inc. New York, New York. 1218 pp. index
• Briggs, D. and S. M. Walters. 1997. Plant
  variation and evolution, 3rd ed. Cambridge
  University Press, Cambridge, United Kingdom. 512
  pp.
• Futuyma, D. J. 1979. Evolutionary biology.
  Sinauer Associates, Inc., Sunderland,
  Massachusetts. 565 pp.
• Gill, D. E. 1986. Individual plants as genetic
  mosaics Ecological organisms versus evolutionary
  individuals. Pp. 321-344 in M. J. Crawley (ed.).
  Plant ecology. Blackwell Scientific, Boston,
  Massachusetts.

25
Bibliography

• Grant, V. 1991. The evolutionary process, 2nd ed.
  Columbia University Press, New York, New York.
  487 pp.
• Suzuki, D. T., A. J. F. Griffiths, J. H. Miller,
  and R. C. Lewontin. 1989. An introduction to
  genetic analysis, 4th ed. W. H. Freeman and
  Company, New York, New York. 768 pp.
• Whitham, T. G. and C. N. Slobodchikoff. 1981.
  Evolution by individuals, plant-herbivore
  interactions, and mosaics of genetic variability
  The adaptive significance of somatic mutations in
  plants. Oecologia 49287-292.