Download The genetics, ecology, and evolution of the Hawaiian silversword

The ecology,
evolution, and
genetics of the
Hawaiian
Silversword
Alliance
Kurt Hartman
Silversword researchers
Gerald
Carr
Robert
Robichaux
Bruce
Baldwin
Silversword
researchers
Bruce
Bohm
Donald
W. Kyhos
Leslie
Bohm
Bill Crins
Adaptive Radiation on Hawaii
Bidens menziesii
Geranium cuneatum
Delissea undulata
Campanulaceae
Adaptive Radiation on Hawaii
honeycreeper
Drosophilidae
What is the Silversword
Alliance?
• 28 species endemic to
Hawaii
• 3 genera
– Argyroxiphium
(5 spp.)
– Dubautia (21 spp.)
– Wilkesia (2 spp.)
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
Silversword species (3)
A. sandwicense
A. kauense
A. caliginis Bog
silversword
2 subspecies of A. sandwicence
ssp. sandwicense
ssp. macrocephalum
Argyroxiphium
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
Silversword trichomes
(Melcher et. al 1994)
Unusual features
• Pectic warts
• Secondary growth
Greenswords (2)
Argyroxiphium
virescens hybrid
Argyroxiphium grayanum
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)
Genus #2 - Wilkesia
•2 species: Wilkesia gymnoxiphium &
Wilkesia hobdyi
W. gymnoxiphium
– grows on pockets
in eastern Kaua’i
(oldest high
island) and may
prefer certain soil
types
– Dry, shrubby
forest
Wilkesia gymnoxiphium
– Seldom branching
stems (branch if
injured), ≳ 10 ft
– Flat, fibrous leaves in
whorls that are strictly
parallel with few
crosscutting veinlets
– Monocarpic
Wilkesia gymnoxiphium
Wilkesia gymnoxiphium
Inflorescence with whorls of
heads, all discoid flowers
Wilkesia hobdyi
Freely branching, decumbent to
erect, endangered, grows only
on Kaua’i, restricted to very dry
ridges (75 - 100cm ppt / yr,
elevation 275 - 400m)
Wilkesia hobdyi
Genus #3 - Dubautia
• Most “recent” genus
• 21 species with differential spatial and
habitat distribution
• Found on Kaua’i, O’ahu, Moloka’i, Lana’i,
Maui, Hawai’i
• 2 other mainland islands have no Dubautia
• 17 out of 21 species of Dubautia are singleisland endemics
D. ciliolata
D. plantaginea
D. latifolia
D. scabra
Movie Part I
Dubautia species
D. latifolia (vine)
– “reticulate vein
Veination patterns in
Dubautia and
Wilkesia
pattern with
polygonal areoles
containing numerous
free-terminating
veins”
D. microcephala (left)
D. linearis (right)
W. gymnoxiphium
(Carlquist 1959)
Dubatia
herbstobatae
Dubatia arborea
Dubautia menziesii - kupaoa
Dubautia waialealae
Dubatia latifolia (vine)
D. laevigata
Dubautia laevigata
Dubautia pauciflorula
2 studies in conservation
Friar et. al 2000
A. sandwicense in bad shape
Friar et. al 2001
A. kauense in good shape
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)
Physiological and morphological divergences
1. Different tissue and elastic properties to maintain
turgor at low water potentials (Robichaux 1985)
2. Different cell structures - especially Dubautia,
a. Wet environments – Thin cuticle, thin leaves, loose
mesophyll
b. Dry environments – Thick cuticle, thick leaves,
compact mesophyll, extracellular mesophyll, and
white hairs (Carlquist 1958)
3. Veination –Dubautia latifolia – highly reticulate;
Wilkesia – monocot-like veination with few
crosscutting veinlets; Others – subparallel or
longitudinally directed veins (Givnish & Sytsma 1997)
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)
What’s in a name? – “Tarweeds”
Tarweed
Raillardiopsis muirrii
A. sandwicense
The original silversword was a
polyploid… what kind of
polyploid?
Hypothesis for polypolidy (n = 14)
Hypothesis for polypolidy (n = 14)
Best explanation
In what way did the silverswords
move around Hawaii in terms of
biogeography and habitats?
Biogeography & phylogeny
• Minimum inter-island dispersal and large
ecological divergence
• Generally westward to eastward movement
• Kaua’i, Oahu, Maui Nui – once contiguous
islands facilitated this movement
How do we investigate these
evolutionary relationships?
Laboratory methods of
investigation
•
•
•
•
•
•
Cytogenetic & hybridization analysis
Nuclear DNA (nDNA)
Chloroplast DNA (cpDNA)
Mitochondrial DNA (mDNA)
Ribosomal DNA (rDNA)
Isozymic analysis
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)
Q: How fast do silverswords evolve
relative to continental groups?
• Kure 29 mya = oldest island
• Actually Compositae is more recent (midoligocene 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)
• 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 Kaua’I (5.1 ±
0.2 mya; is the oldest high island)
• Other species are older on Hawaii
– Drosophila > 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
• 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
30
no. of species
25
20
15
10
5
0
-0.5
0.5
1.5
2.5
3.5
4.5
5.5
3.5
4.5
5.5
millions of years
3.5
no. of species (ln)
3
2.5
2
1.5
1
0.5
0
-0.5
0.5
1.5
2.5
millions of years
Hybrids
• Today’s silverswords are polyploid
• Most silverswords are n=14 with few n=13
in Dubautia subgroup
• Was original ancestor a hybrid derived from
allopolyploid or autopolyploid individual?
Movie Part II
Hybrids
Hybridization
Trigeneric hybrid
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.
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
Summary
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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
The End