The genus Wolffiella also resolves consistently
as monophyletic. Plants in the genus have flattened fronds and flowers that are located laterally
on the upper frond surface. Sections of Wolffiella
largely have been upheld by molecular data and
are morphologically distinct. Section Stipitatae
(W. hyalina and W. repanda) includes species
having an elongated appendage that originates
from the vegetative budding pouch. Sections
Stipitatae and Rotundae, the latter comprising
only W. rotunda, together are monophyletic and
contain the only Wolffiella species that have
smaller (<3 mm), orbicular to ovate fronds that
float on the water surface. Section Wolffiella also
is monophyletic and contains all other species in
the genus, in contrast to sections Rotundae and
Stipitatae; species in section Wolffiella have
slender, elongated fronds (>3 mm) that grow at
least partially submersed.
2.5 Biogeography
Several duckweed species have broad distributions that extend over multiple continents (e.g., S.
polyrhiza,
Landoltia
punctata,
Lemna
aequinoctialis, L. minor, L. trisulca), whereas
others have narrower ranges (e.g., Lemna tenera,
Wolffia elongata, Wolffiella denticulata; Fig. 2.2
a–e). Les et al. (2003) evaluated pairs of closely
related species and concluded that duckweeds, as
well as numerous other aquatic plant groups,
effected transoceanic dispersal in the relatively
recent past. Duckweeds are capable of external
biotic dispersal (i.e., exozoochory), which they
accomplish by adhering to the surfaces of ducks
and other waterfowl that are known to travel
large distances (Jacobs 1947; Hillman 1961;
Kimball et al. 2003; Coughlan et al. 2014).
Moreover, their habit of growing over large areas
of water surface makes it relatively easier for
them to attach to a biotic vector, and their
extremely small size and propensity for vegetative reproduction increase the likelihood that
populations will establish in new territory
(Coughlan et al. 2017; Green 2016).
In a phylogenetic context, it becomes apparent
just how many times duckweeds have established
in novel locations relative to their immediate
ancestors. We conducted a biogeographic analysis in RASP (reconstruct ancestral state in
phylogenies) ver. 3.2 (Yu et al. 2015), with the
S-DIVA reconstruction (Yu et al. 2010), using
geographic regions that commonly are not
occupied by the same duckweed species (i.e.,
Americas north of Yucatán; Americas south of
Yucatán; Africa; Eurasia west of India and north
of Japan/Korea; eastern Eurasia including India,
Japan, Korea, and Malaysia; and Australia and
New Zealand; Fig. 2.1), to reconstruct the colonization history of duckweed species. The
reconstruction was constrained to allow a maximum of two areas to be occupied simultaneously.
Dispersal events were plotted onto an ultrametric
tree that was calibrated using the duckweed
crown age of 73.4 Ma. In their biogeographic
analysis of Araceae, Nauheimer et al. (2012)
recognized the dispersal ability of aquatic taxa
and analyzed duckweeds as having a “water
associated” range with high dispersal ability.
They reconstructed the common ancestor of all
Araceae to be North American, whereas the
sibling lineage of duckweeds was reconstructed
to have a Eurasian ancestor. Fossil duckweed
relatives recovered from late Cretaceous formations in North America (Kvaček 1995; Stockey
et al. 1997, 2007, 2016) may represent extinct
lineages that predated the clade of extant duckweeds, and it is noteworthy that contemporary
fossils putatively related to duckweeds also have
been recovered from Africa (Coiffard and Mohr
2018).
In our analysis, the phylogenetic positions of
S. polyrhiza and Landoltia punctata, two species
with worldwide distributions (Fig. 2.2a), along
with the disparate distribution ranges of related
Araceae lineages, resulted in broad uncertainty
about the geographic range of the duckweed
ancestor (Fig. 2.1). The common ancestor of
Lemna species (41.7 Ma [crown age]—54.4 Ma
[stem age]) was reconstructed with highest
probability to be in North America. Within genus
Lemna, section Uninerves was reconstructed to
have an ancestor in the Americas (2.7–34.8 Ma),
where all three species are native (Fig. 2.2b). The
ancestor of sections Alatae and Biformes likely
28
N. P. Tippery and D. H. Les
as monophyletic. Plants in the genus have flattened fronds and flowers that are located laterally
on the upper frond surface. Sections of Wolffiella
largely have been upheld by molecular data and
are morphologically distinct. Section Stipitatae
(W. hyalina and W. repanda) includes species
having an elongated appendage that originates
from the vegetative budding pouch. Sections
Stipitatae and Rotundae, the latter comprising
only W. rotunda, together are monophyletic and
contain the only Wolffiella species that have
smaller (<3 mm), orbicular to ovate fronds that
float on the water surface. Section Wolffiella also
is monophyletic and contains all other species in
the genus, in contrast to sections Rotundae and
Stipitatae; species in section Wolffiella have
slender, elongated fronds (>3 mm) that grow at
least partially submersed.
2.5 Biogeography
Several duckweed species have broad distributions that extend over multiple continents (e.g., S.
polyrhiza,
Landoltia
punctata,
Lemna
aequinoctialis, L. minor, L. trisulca), whereas
others have narrower ranges (e.g., Lemna tenera,
Wolffia elongata, Wolffiella denticulata; Fig. 2.2
a–e). Les et al. (2003) evaluated pairs of closely
related species and concluded that duckweeds, as
well as numerous other aquatic plant groups,
effected transoceanic dispersal in the relatively
recent past. Duckweeds are capable of external
biotic dispersal (i.e., exozoochory), which they
accomplish by adhering to the surfaces of ducks
and other waterfowl that are known to travel
large distances (Jacobs 1947; Hillman 1961;
Kimball et al. 2003; Coughlan et al. 2014).
Moreover, their habit of growing over large areas
of water surface makes it relatively easier for
them to attach to a biotic vector, and their
extremely small size and propensity for vegetative reproduction increase the likelihood that
populations will establish in new territory
(Coughlan et al. 2017; Green 2016).
In a phylogenetic context, it becomes apparent
just how many times duckweeds have established
in novel locations relative to their immediate
ancestors. We conducted a biogeographic analysis in RASP (reconstruct ancestral state in
phylogenies) ver. 3.2 (Yu et al. 2015), with the
S-DIVA reconstruction (Yu et al. 2010), using
geographic regions that commonly are not
occupied by the same duckweed species (i.e.,
Americas north of Yucatán; Americas south of
Yucatán; Africa; Eurasia west of India and north
of Japan/Korea; eastern Eurasia including India,
Japan, Korea, and Malaysia; and Australia and
New Zealand; Fig. 2.1), to reconstruct the colonization history of duckweed species. The
reconstruction was constrained to allow a maximum of two areas to be occupied simultaneously.
Dispersal events were plotted onto an ultrametric
tree that was calibrated using the duckweed
crown age of 73.4 Ma. In their biogeographic
analysis of Araceae, Nauheimer et al. (2012)
recognized the dispersal ability of aquatic taxa
and analyzed duckweeds as having a “water
associated” range with high dispersal ability.
They reconstructed the common ancestor of all
Araceae to be North American, whereas the
sibling lineage of duckweeds was reconstructed
to have a Eurasian ancestor. Fossil duckweed
relatives recovered from late Cretaceous formations in North America (Kvaček 1995; Stockey
et al. 1997, 2007, 2016) may represent extinct
lineages that predated the clade of extant duckweeds, and it is noteworthy that contemporary
fossils putatively related to duckweeds also have
been recovered from Africa (Coiffard and Mohr
2018).
In our analysis, the phylogenetic positions of
S. polyrhiza and Landoltia punctata, two species
with worldwide distributions (Fig. 2.2a), along
with the disparate distribution ranges of related
Araceae lineages, resulted in broad uncertainty
about the geographic range of the duckweed
ancestor (Fig. 2.1). The common ancestor of
Lemna species (41.7 Ma [crown age]—54.4 Ma
[stem age]) was reconstructed with highest
probability to be in North America. Within genus
Lemna, section Uninerves was reconstructed to
have an ancestor in the Americas (2.7–34.8 Ma),
where all three species are native (Fig. 2.2b). The
ancestor of sections Alatae and Biformes likely
28
N. P. Tippery and D. H. Les