Plant Speciation

1
Plant Speciation Evolution (PBIO 475/575)

• Breeding Systems and Reproductive Strategies

2
Breeding Systems in General

• Broad categoriesendpoints of a continuum
• Xenogamy (outcrossing)--mating between different,
  ideally unrelated individuals
  
• Autogamy (selfing)--mating within a flower on an
  individual
  

3
Breeding Systems in General

• Other "variations" accomplish same end
• Allogamy--mating between flowers on same plant
• Geitonogamy--mating between vegetative clones of
  original "mother" plant or with same genotype
  (inbred lines)
  
• Extremes run counter to one another in
  population-level evolutionary consequences

4
Breeding Systems in General

• Many intermediary systems
• One BS more successful than another under
  particular circumstances

Stuessy (1990)
5
Breeding Systems in General

• Outcrossing maintains high genetic variation for
  unpredictable environments but tends to break up
  locally adapted allele combinations (gene
  complexes)
• Selfing and inbreeding maintain genotypes best
  adapted to predictable microsites (e.g., those
  near parent plant) but don't provide broad
  variation needed to cope with longer-term
  environmental change
• Thousands of plant species across ca. 60
  angiosperm families utilize 2 or more breeding
  systems ? evolutionarily flexible strategy

6
Breeding Systems in General

• Breeding system variation largely peculiar to
  plants
  

Stuessy (1990)
7
Breeding Systems in General

• Most animals have sexes separated on two
  individuals (dioecious)
  
• Most plants (97) have naughty bits on one body
  (hermaphroditic or monoecious)
  
• Incidence of dioecy (sexes on separate plants)
  varies with world region and setting--low in
  northern Europe and North America but increases
  substantially in oceanic islands and woody
  lowland tropical plants

8
Outcrossing Mechanisms

• First investigated by Darwin
• made numerous observations on many different
  species
  
• noted how floral architecture, different
  maturation times of anthers (pollen sacs) and
  stigma could affect cross-pollination
  
• Morphological, physiological and biochemical
  adaptations accumulate to promote outcrossing in
  most species

9
Outcrossing Mechanisms

• Self-incompatibility systems
• Pollen cannot germinate on stigma of same flower
  or same plant
  
• gametophyticpollen germination depends on
  genotype of style
  
• sporophyticpollen germination depends on
  genotype of pollen parent

Briggs and Walters (1997)
10
Outcrossing Mechanisms

• Self-incompatibility systems (cont.)
• Common in species that cannot produce progeny by
  selfing, although self-pollination may occur
  regularly
  
• biochemically regulated at pollen-stigma
  interface or between elongating pollen tube and
  stylar tissue
  
• genetically determined by complex systems of
  multiple alleles present in maternal plant and
  pollen

11
Outcrossing Mechanisms

• Heterostyly
• Found in entomophilous species
• Different floral forms on different individuals
• Each form differs from others in relative
  position of stigma and anthers
  
• Pollen transfer only successful with visits
  between forms

Briggs and Walters (1997)
12
Outcrossing Mechanisms

• Distyly and tristyly both relatively common in
  diverse angiosperm groups
  
• Classic studies on distyly made on primroses
  (Primula), on tristyly with loosestrife
  (Lythrum)
  
• Sometimes more complex, with different pollen
  types as well!
  

Briggs and Walters (1997)
13
Outcrossing Mechanisms

• Heterostyly and associated incompatibility
  reaction controlled by "supergenes
  
• In distyly, thrum plants are heterozygous
  (GPA/gpa), pin plants 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)
  

14
Selfing

• Requires self-compatibility
• Often accompanied by structurally enforced
  mechanism ( cleistogamous flowers)
  
• Many plant species across angiosperms self (over
  60 families), has evolved repeatedly from
  outcrossing ancestors
  
• Some species are obligate selfers (e.g., some
  grasses, some orchids), others produce
  self-fertilizing flowers seasonally, etc.

15
Asexual Breeding Systems

• Vegetative apomixis
• Reproduction entirely by vegetative propagation
  by plant fragments, bulbils, etc.
  
• Common in aquatic plants, woody plants, vines,
  rhizomatous herbs
  
• e.g., waterweed (Elodea) and horsetail
  (Equisetum) fragment and disperse stem pieces
  
• e.g., cottonwood (Populus) and sumac (Rhus) form
  extensive underground root systems, which
  fragment
  
• e.g., duckweed (Lemna) buds off pieces of
  thallus that regenerate new plants

16
Asexual Breeding Systems

• Vegetative apomixis (cont.)
• Viviparygermination of fruits or other
  propagules while still attached to maternal
  parent
  
• true viviparypropagule is product of sexual
  reproduction
  
• e.g., red mangrove (Rhizophora mangle)
• pseudoviviparypropagule arises asexually
• e.g., alpine bistort (Polygonum
  viviparum)--plantlets drop off and grow into new
  genetically identical plants
  
• e.g., many onions (Allium), with inflorescence
  producing flowers and vegetative bulblets that
  drop and germinate

17
Asexual Breeding Systems

• Agamospermy
• Normal seed set but without sex--normal
  embryo-sac generates diploid egg cell that does
  not undergo meiosis
  
• ? progeny genetically like parent
• e.g., dandelion (Taraxacum) sets seed when
  flowers are bagged--no pollination necessary
  

18
Asexual Breeding Systems

• Agamospermy (cont.)
• Pseudogamy--a "deviant" situation where
  pollination is needed to initiate agamospermous
  seed production
  
• Very widespread in angiosperms (e.g., sunflower
  family, rose family)
  
• Must test for this by conducting embryological
  and genetic studiesapparently normal sexual
  reproduction might actually be pseudogamy

19
Asexual Breeding Systems

• Apogamy--diploid vegetative cells of gametophyte
  produce embryo-sac
  
• common in ferns and angiosperms
• apparently absent in gymnosperms
• Apospory--gametophyte arises from unreduced
  (diploid) megaspore or somatic cell of sporophyte

20
Mixed Breeding Systems

• Wide range of combinations
• May include predominately outcrossing systems,
  mostly selfing ones, apomictic ones, or all
  three
  
• Widespread across vascular plants
• Considered a highly flexible and successful
  evolutionary strategy

21
Mixed Breeding Systems

• Case 1--outcrossing and vegetative apomixis
• e.g., clover (Trifolium)reproducing by
  outcrossing flowers, clonal fragments of branched
  reclining or prostrate stems
  
• e.g., onion (Allium)--reproducing by subterranean
  bulbs (division by bulbs), inflorescence bulblets
  (pseudovivipary), outcrossing flowers
  

22
Mixed Breeding Systems

• Case 2outcrossing, self-pollination, strict
  autogamy (and apomixis in some species)
  
• e.g., violets (Viola)most species outcross, are
  self-compatible, and produce cleistogamous
  flowers
  
• some species also produce stolons or fragmenting
  rhizomes!
  

Raven et al. (1992)
23
Bibliography

• Briggs, D. and S. M. Walters. 1997. Plant
  variation and evolution, 3rd ed. Cambridge
  University Press, Cambridge, United Kingdom. 512
  pp.
• Elmqvist, T. and P. A. Cox. The Evolution of
  vivipary in flowering plants. Oikos 77(1) 3-9.
  
• Raven, P. H., R. F. Evert, and S. E. Eichhorn.
  1992. Biology of plants, 5th ed. Worth
  Publishers, New York, New York. 791 pp.
  
• Stuessy, T. 1990. Plant taxonomy The systematic
  evaluation of comparative data. Columbia
  University Press, New York, New York. 514 pp.