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 Polypodium3. Peripatric speciation in Pleopeltis4. 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 angustum asplenioides

Rhizome habit erect or ascending ascending or creepingLeaf shape tapered at base barely tapered at baseSpore color yellowish brownish-yellow to

blackSpore surface features sparsely papillate wrinkled or reticulated

A. f.-f. subsp. angustum A. f.-f. subsp. asplenioides

At the four most polymorphic loci (Idh-1, Pgm-2, Pgi-2, and Tpi-2) population allele frequencies were quite similar within taxon, but quite different between taxa.

Isozyme Results




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: 223-

236.

Polypodium virginianum(4x)

Polypodium appalachianum

(2x)

Polypodium sibiricum(2x)

Polypodium amorphum(2x)

Mean genetic identitybetween 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 did not take place in the most recent ice age.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 allopatricallopatricmodel.




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 ofthere may be strong selection for the evolution of post-zygotic isolating mechanisms.post-zygotic isolating mechanisms.

Pleopeltis sp.

Pleopeltis polypodioides

Mean genetic identitybetween species = 0.849

Why the difference? Why the difference?

Perhaps the mode of speciation hereis peripatric, including peripheral isolation and rapid adaptation to differing habitats.

The greater complexity of tropical habitats may provide increased opportunities for ecological isolationecological isolation and speciation.




Yatabe on the Asplenium nidus complex

Yoko Yatabe with Kato and Gong Wu su. China 2001


Yatabe et al. 2009


Yatabe et al. 2009


Yatabe et al. 2009


Yatabe et al. 2009


Yatabe et al. 2009


Yatabe et al. 2009


Yatabe et al. 2009




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Divergent Evolution in the Ferns