The ecology, evolution, and genetics of the Hawaiian Silversword Alliance

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The ecology, evolution, and genetics of the
Hawaiian Silversword Alliance

• Kurt Hartman

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Silversword researchers
Gerald Carr
Bruce Baldwin
Robert Robichaux
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Silversword researchers
Leslie Bohm
Bruce Bohm
Donald W. Kyhos
Bill Crins
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Adaptive Radiation on Hawaii
Bidens menziesii
Geranium cuneatum
Delissea undulata Campanulaceae
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Adaptive Radiation on Hawaii
honeycreeper
Drosophilidae
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What is the Silversword Alliance?

• 28 species endemic to Hawaii
• 3 genera
• Argyroxiphium
• (5 spp.)
• Dubautia (21 spp.)
• Wilkesia (2 spp.)

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Genus 1 - Argyroxiphium the silverswords
greenswords

• Most recognized genus of silversword alliance
• Capitulum with ray flowers (contrast with
  Wilkesia Dubautia which only have disk flowers)
• Hybrid between Argyroxiphium, Wilkesia, and
  Dubautia species
• Found on Maui and Hawaii

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Silversword species (3)
A. caliginis Bog silversword
A. kauense
A. sandwicense
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2 subspecies of A. sandwicence

• ssp. sandwicense

ssp. macrocephalum
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Argyroxiphium
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Argyroxiphium sandwicense habitat

• Habitat on cinder cone
• 7000 to 10000 feet intense sunlight
• Hot in summer day below freezing in winter
  night
• Snowfall, zero humidity, low ppt alpine desert

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Silversword trichomes
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(Melcher et. al 1994)
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Unusual features

• Pectic warts
• Secondary growth

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Greenswords (2)
Argyroxiphium virescens hybrid
Argyroxiphium grayanum
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Changes in soil characteristics below
Argyroxiphium

• Soil below silversword increased in water and
  nutrient retention, lower temperature (18C),
  greater nutrient concentration for up to 7-9 yrs
  post mortum.
• Therefore good for current survival and future
  offspring who may be close in proximity to
  parent.
• (Perez 2001)

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Genus 2 - Wilkesia

• 2 species Wilkesia gymnoxiphium Wilkesia
  hobdyi

• W. gymnoxiphium
• grows on pockets in eastern Kauai (oldest high
  island) and may prefer certain soil types
• Dry, shrubby forest

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Wilkesia gymnoxiphium

• Seldom branching stems (branch if injured), ? 10
  ft
• Flat, fibrous leaves in whorls that are strictly
  parallel with few crosscutting veinlets
• Monocarpic

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Wilkesia gymnoxiphium
Wilkesia gymnoxiphium
Inflorescence with whorls of heads, all discoid
flowers
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Wilkesia hobdyi
Freely branching, decumbent to erect, endangered,
grows only on Kauai, restricted to very dry
ridges (75 - 100cm ppt / yr, elevation 275 -
400m)
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Wilkesia hobdyi
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Genus 3 - Dubautia

• Most recent genus
• 21 species with differential spatial and habitat
  distribution
• Found on Kauai, Oahu, Molokai, Lanai, Maui,
  Hawaii
• 2 other mainland islands have no Dubautia
• 17 out of 21 species of Dubautia are single-
  island endemics

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D. ciliolata

• D. plantaginea

D. latifolia
D. scabra
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Movie Part IDubautia species
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D. latifolia (vine) reticulate vein pattern
with polygonal areoles containing numerous
free-terminating veins

• Veination patterns in Dubautia and Wilkesia

D. microcephala (left) D. linearis
(right)
W. gymnoxiphium
(Carlquist 1959)
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Dubatia herbstobatae
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Dubatia arborea
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Dubautia menziesii - kupaoa
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Dubautia waialealae
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Dubatia latifolia (vine)
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D. laevigata
Dubautia laevigata
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Dubautia pauciflorula
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2 studies in conservationFriar et. al 2000A.
sandwicense in bad shape Friar et. al 2001A.
kauense in good shape
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Habitat divergence

• Elevation from 75 to 3750m
• Habitats of dry shrublands, dry forests,
  subalpine shrublands, subalpine forests, alpine
  deserts, mesic forests, wet forests, bogs, young
  lava flows (Carr 1985)

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Physiological and morphological divergences

• Different tissue and elastic properties to
  maintain turgor at low water potentials
  (Robichaux 1985)
• Different cell structures - especially Dubautia,
  
• Wet environments Thin cuticle, thin leaves,
  loose mesophyll
• Dry environments Thick cuticle, thick leaves,
  compact mesophyll, extracellular mesophyll, and
  white hairs (Carlquist 1958)
• Veination Dubautia latifolia highly
  reticulate Wilkesia monocot-like veination
  with few crosscutting veinlets Others
  subparallel or longitudinally directed veins
  (Givnish Sytsma 1997)

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Who are the ancestors of silversword alliance?

• Tarweeds found in California floristic province
  CA and Mex.
• Shrubby, sprawling plants
• Similar in floral morphology and anatomy to
  silverswords
• Sticky substance on flowers and fruits
• Arrived in Hawaii on bird feathers (most likely)
  roughly 5 mya (Baldwin Robichaux 1995)

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Whats in a name? Tarweeds
Tarweed
A. sandwicense
Raillardiopsis muirrii
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The original silversword was a polyploid what
kind of polyploid?
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Hypothesis for polypolidy (n 14)
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Hypothesis for polypolidy (n 14)
Best explanation
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In what way did the silverswords move around
Hawaii in terms of biogeography and habitats?
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Biogeography phylogeny

• Minimum inter-island dispersal and large
  ecological divergence
• Generally westward to eastward movement
• Kauai, Oahu, Maui Nui once contiguous islands
  facilitated this movement

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How do we investigate these evolutionary
relationships?
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Laboratory methods of investigation

• Cytogenetic hybridization analysis
• Nuclear DNA (nDNA)
• Chloroplast DNA (cpDNA)
• Mitochondrial DNA (mDNA)
• Ribosomal DNA (rDNA)
• Isozymic analysis

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Q How do silverswords evolve so fast?

• Problem rates of morphological evolution are
  generally not correlated with rates of molecular
  evolution how is this possible?
• ASAP3/TM6 controls petal and stamen development
• ASAP 1 controls floral primordia and sepal and
  petal identity
• Rapid regulatory gene mutations relative to the
  number of mutations of structural genes.
• (Baldwin Sanderson 1998)

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Q How fast do silverswords evolve relative to
continental groups?

• Kure 29 mya oldest island
• Actually Compositae is more recent (mid-oligocene
  ca. 25 mya)
• Shift about 15 mya from wet summer to dry summer
• Tarweeds begin to diverge
• Used ITS (internal transcribed spacer) region of
  nuclear DNA
• Excluded annuals in analysis b/c higher rate of
  evolution
• (Baldwin Sanderson 1998)

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• Found silverswords are well nested in tarweeds
  because left N. America after tarweeds diverged
• Today tarweeds ca. 114 species in 17 genera
• Estimated age of arrival in Hawaii to be 5.2
  0.8 mya which is age of KauaI (5.1 0.2 mya is
  the oldest high island)

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• Other species are older on Hawaii
• Drosophila gt 10 mya
• Lobelioids 15 mya
• Honeycreepers 7-8 or 15-20 mya
• Older ancestor is possible from N. Am, but only
  one lineage survived at 5.2 mya

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• Divergence rate is 0.56 0.17 species / million
  yrs
• Angiosperm families (0.12 species / million yrs)
• Rodent families (0.22 to 0.35 species / million
  yrs)
• African large mammals (0.0 to 0.39 species /
  million yrs)
• However early Neocene horses show 0.5 to 1.4
  species / million yrs) which indicates fast
  radiation then slow. Similar to above rate for
  silverswords.
• Likely that early radiation is very fast then
  slows

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Hybrids

• Todays silverswords are polyploid
• Most silverswords are n14 with few n13 in
  Dubautia subgroup
• Was original ancestor a hybrid derived from
  allopolyploid or autopolyploid individual?

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Movie Part IIHybrids
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Hybridization
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Trigeneric hybrid
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Study Carraway et. al 2001

• D. ciliolata 1855 lava flow
• D. scabra 1935 lava flow
• Hybrids and introgression of only D. ciliolata.
• D. ciliolata genes in hybrid swarm able to
  colonize 1935 lava flow.
• I.e. occupy new habitat with hybridization and
  genetic restructuring.

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Why is silversword alliance a textbook example
of adaptive radiation? (Raven et. al 1992)

• Includes ecology long distance and local
  dispersal morphology physiology and adaptation
• Includes evolution phylogeny hybridization
  rates of evolution
• Includes genetics cpDNA nrDNA rDNA isozyme
  congruencies and incongruencies
• Conservation population dynamics extinction
  positive and negative human influences

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Summary

• Single colonist (allopolyploid) 3 genera today
• Biogeographical movement west to east (mostly)
• Most are single-island endemics
• Radiate to different habitats on islands
• Morphological and physiological adaptation
• Systematics are well studied
• Needs protection from humans and grazing

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The End