Transcript Divergent Evolution in the Ferns

Divergent Evolution in the Ferns
Botany 209
16 March, 2010
DIVERGENT SPECIATION in the FERNS
1. Incomplete divergence (versus secondary contact)
in Athyrium filix-femina
2. Allopatric speciation in Polypodium
3. Peripatric speciation in Pleopeltis
4. Species limits and speciation in Asplenium nidus
Phylogeny of Isoetes species based on ITS sequence data
Kelloff et al. 2002. Differentiation of eastern
North American Athyrium filix-femina taxa:
evidence from allozymes and spores. Amer.
Fern Journal 92:185-213.
Athyrium filix-femina subsp. angustum
Athyrium filix-femina subsp. asplenioides
Character
Rhizome habit
Leaf shape
Spore color
angustum
erect or ascending
tapered at base
yellowish
Spore surface features sparsely papillate
asplenioides
ascending or creeping
barely tapered at base
brownish-yellow to
black
wrinkled or reticulated
A. f.-f. subsp. angustum
A. f.-f. subsp. asplenioides
Isozyme Results
At the four most polymorphic loci (Idh1, Pgm-2, Pgi-2, and Tpi-2) population
allele frequencies were quite similar
within taxon, but quite different
between taxa.
DIVERGENT SPECIATION in the FERNS
1. Incomplete divergence (versus secondary contact)
in Athyrium filix-femina
2. Allopatric speciation in Polypodium
3. Peripatric speciation in Pleopeltis
4. Species limits and speciation in Asplenium nidus
Lisa Andrews Hooper
Chris Haufler
Haufler, C. H., E. A. Hooper, and J. P. Therrien. 2000.
Modes and mechanisms of speciation in pteridophytes:
implications of contrasting patterns in ferns representing
temperate and tropical habitats. Plant Species Biology 15: 223236.
Polypodium sibiricum
(2x)
Polypodium
appalachianum
(2x)
Polypodium virginianum
(4x)
Polypodium amorphum
(2x)
Mean genetic identity
between species =
0.460
Haufler’s divergence scenario for Polypodium:
 P. sibiricum occurs in glaciated regions. Perhaps,
like other boreal taxa, P. sibiricum populations
migrated south in advance of the ice sheet.
 Following retreat of the ice, populations returned to
their northern haunts, but perhaps some populations
persisted in the mountains of eastern and western North
America. Isolation and divergence perhaps led to
speciation of P. amorphum in the west and
P. appalachianum in the east.
The low I values for the three species suggest that
these events did not take place in the most recent
ice age. In fact, secondary contact following
southerly migration of P. sibiricum during more
recent episodes of glaciation may have initiated both
P. virginianum and P. saximontanum, a western tetraploid.
The mode of speciation in the P. sibiricum
complex is thus fits the classic allopatric
model.
DIVERGENT SPECIATION in the FERNS
1. Incomplete divergence (versus secondary contact)
in Athyrium filix-femina
2. Allopatric speciation in Polypodium
3. Peripatric speciation in Pleopeltis
4. Species limits and speciation in Asplenium nidus
So why the low identities among congeneric
species in the ferns?
 One possibility is that they diverged long ago.
 Another is that, in the absence of pre-zygotic isolating
mechanisms (pollination biology, pollen-stigma interactions,
different flowering times, etc.) to maintain barriers between
species, there may be strong selection for the evolution of
post-zygotic isolating mechanisms.
Pleopeltis sp.
Pleopeltis polypodioides
Mean genetic identity
between species =
0.849
Why the difference?
Perhaps the mode of speciation here
is peripatric, including peripheral isolation
and rapid adaptation to differing habitats.
The greater complexity of tropical habitats
may provide increased opportunities for
ecological isolation and speciation.
DIVERGENT SPECIATION in the FERNS
1. Incomplete divergence (versus secondary contact)
in Athyrium filix-femina
2. Allopatric speciation in Polypodium
3. Peripatric speciation in Pleopeltis
4. Species limits and speciation in Asplenium nidus
Yatabe on the Asplenium nidus complex
Yoko Yatabe with Kato and Gong Wu su. China 2001
Yatabe on the Asplenium nidus complex
Yatabe et al. 2009
Yatabe on the Asplenium nidus complex
Yatabe et al. 2009
Yatabe on the
Asplenium
nidus complex
Yatabe et al. 2009
Yatabe on the
Asplenium nidus
complex
Yatabe et al. 2009
Yatabe on the Asplenium nidus complex
Yatabe et al. 2009
Yatabe on the
Asplenium
nidus complex
Yatabe et al. 2009
Yatabe on the Asplenium nidus complex
Yatabe et al. 2009
DIVERGENT SPECIATION in the FERNS
1. Incomplete divergence (versus secondary contact)
in Athyrium filix-femina
2. Allopatric speciation in Polypodium
3. Peripatric speciation in Pleopeltis
4. Species limits and speciation in Asplenium nidus