Revue internationale d'écologie méditerranéenne International ...

LAURENT HARDION, ALEX BAUMEL, PIERRE-JEAN DUMAS, NATHALIE DUONG, LAURENCE AFFRE & THIERRY TATONI
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Discussion
Astragal phylogenies
and Astragalus tragacantha place
The nrDNA ITS phylogeny presented here
supports the existence of four clades within
Astragalus sensu stricto. This phylogeny is
congruent with former molecular works on
astragals (Wojciechowski et al. 1999; Kazempour
Osaloo et al. 2005), and few dissimilarities
have been noticed, most of them corresponding
to nodes supported by weak
bootstrap value. In order to clarify our work,
we re-use well supported clade letters from
these previous studies. All these groups contain
astragals of different sub-genera and sections.
Clade A is the most basal of Astragalus
sensu stricto. It gathers particularly species of
subgenera Calycophysa (e.g. Astragalus
alopecurus) and Phaca (e.g. A. vulcanicus).
Based on its composition in previous molecular
studies (Kazempour Osaloo et al. 2003),
Clade B is poorly represented here because its
resolution is not important for our objectives.
Astragalus tragacantha is located in the clade
C with twelve other species of three different
subgenera (Cercidothrix, Trimeniaeus and
Phaca). This place is confirmed by sequencing
of the chloroplast ndhF gene (data not
shown). Astragalus glycyphyllos, a widely
distributed Eurasian species, takes place at the
basis of this clade, strongly supported by
bootstrap value (96%). Lastly, clade D, the
most sampled and studied (Wojciechowski et
al. 1999; Kazempour Osaloo et al. 2005),
includes the monophyletic group of Neo-
Astragalus which are New World aneuploid
astragals, and most of thorny astragals representing
subgenus Tragacantha (represented
here by A. brachycalyx and A. compactus) and
some species of other subgenera (presented
here by A. lamprocarpus belonging to the
subgenus Phaca).
MP tree and comparison between
Astragalus tragacantha
and its relatives
The strict consensus MP tree based on the
18 sequences of the clade C shows high bootstrap
values. First, Astragalus tragacantha
sequences gather in a monophyletic clade,
continental and Corsican samples forming
two well supported subclades. This phylogenetic
pattern involves a rather ancient occur-
rence of this species in Corsica and supportes
the existence of A. tragacantha ssp. terraccianoi
(Valsecchi 1994), a Corsican endemic
subspecies. Seed dispersal system of A. tragacantha
is mainly barochoric, and thus
seems to not allow long distance dispersion.
Moreover, preliminary molecular dating
analysis using PATHd8 (Britton et al. 2007)
and calibrated upon estimated ages of astragal
clades (Wojciechowski 2005) indicate that
divergences within the clade formed by A. tragacantha,
A. fragrans and A. odoratus would
date from Quaternary (analysis not shown).
This result should be tested by molecular dating
methods combining numerous genes.
Astragalus fragrans and A. odoratus, the closest
relatives of A. tragacantha in this study,
possess a clearly different architecture from a
thorny cushion. These perennial herbaceous
species have no spines and occur around the
Southern border between Europe and Asia
(Anatolia, Caucasia), in altitude between
1200-3050 m and 700-1950 m respectively
(Davis 1970). Other species of clade C show
a wide distribution through Northern Hemisphere.
In fact, even though Astragalus falcatus
occurs in the same zone that the three
precedent species, A. uliginosus is localized
in Central Asia and A. canadensis and A. oreganus
are confined in North America. These
two latter species correspond to another colonization
of North America, different from
that of Neo-Astragalus, an aneuploid group
constituted by most astragals of this continent.
Wojciechowski et al. (1999) supposes a more
recent migration than Neo-Astragalus, via a
Behring land bridge. The wide distribution
and variation of clade 1 taxa (Figure 2) illustrates
the rapid spread and adaptive radiation
of astragals on a global scale as mentioned by
Wojciechowski (2005). Astragalus tragacantha
seems to be the only one living at very low
altitude, near the seashore. Its characteristics,
compared to its non-thorny relatives, could be
explained by adaptation to a stressing habitat.
Among characters investigated here we note
that all members of clade 1 are euploid, like
the majority of Old World astragals, and
perennial. These two criteria distinguish
clearly them from clade 2, which includes
only annual aneuploid astragals of subgenus
Trimeniaeus occurred in Mediterranean
Basin. This pattern is a second particularity
for clade C because majority of aneuploid
astragals are Neo-Astragalus occurring in
America (Wojciechowski et al. 1993). Lastly,
Astragalus glycyphyllos (out-group), a peren-
ecologia mediterranea – Vol. 36 (1) – 2010