Agriculture Reference
In-Depth Information
(Spooner
et al
., 2004), and in a number of mor-
phometric studies, extensive overlap of character
states was observed in many of these characters.
The extremes can be distinguished easily from
each other, but they are connected by a continu-
ous series of intermediate character states, mak-
ing the delimitation of species highly arbitrary.
Two examples of the need for a reduction of spe-
cies numbers are the studies of the so-called
brevicaule
complex (Van den Berg
et al
., 1998;
Miller and Spooner, 1999; Alvarez
et al
., 2008) and
of the Mexican series
Longipedicellata
(Spooner
et al
., 2000; Van den Berg
et al
., 2002). Both
studies relied on extensive morphometric char-
acterization of gene bank accessions, planted in
a common garden plot, supplemented with mo-
lecular data.
The
brevicaule
complex consists of a group
of more than
30
species placed in series
Tuberosa
by Hawkes (1990). These species are the closest
relatives of the cultivated potato, are extremely
similar to each other, and are distributed from
southern Peru, via Bolivia, to northern Argen-
tina, with different species names being applied
in different countries. Hawkes (1990) suggested
that the extreme speciation of this complex in
the Peruvian Andes could be accounted for by
the large number of closed valley systems in that
mountain range, where many populations be-
came relatively isolated after the last glaciation.
Phenetic results (Van den Berg
et al
., 1998) did
not support many distinct taxa in this complex.
Only two or three broadly defined taxa could be
recognized. Miller and Spooner (1999), in a
study of the molecular data of this complex,
concluded that their molecular data concurred
with the data of Van den Berg
et al
. (1998). Two
geographically based groups were indicated, one
from Peru and the other from Bolivia and Argen-
tina. The repeat experiment in Peru (Alvarez
et al
.,
2008) came to the same conclusion: the collapse
of many of the species of the
Solanum brevicaule
complex was unavoidable.
Similarly, a morphological study (Spooner
et al
., 2000), followed by a study of the data from
three molecular markers (amplified fragment
length polymorphism (AFLPs); random ampli-
fied polymorphic DNA (RAPDs); and chloroplast
simple sequence repeats (cpSSRs)) (Van den
Berg
et al
., 2002), supported only two species
in the series
Longipedicellata
, instead of the six
recognized by Hawkes (1990). The morphological
characters that had been used to distinguish
species displayed substantial overlap in their
character states, making equivocal identifications
impossible, especially between
Solanum fendleri
and
S. stoloniferum
. The molecular data did not
separate the species recognized by Hawkes, but
allowed the distinction of
S. stoloniferum
s.l. and
Solanum hjertingii
.
The study also included South American
species that were morphologically similar. The
reproductive isolation of the Mexican
Longipedi-
cellata
species (based on their
4×
(2EBN) crossabili-
ty and AABB genome) from these morphologically
very similar South American species (especially
from series
Tuberosa
) demonstrates a conflict be-
tween a morphological and a biological species
concept. Apparently, morphology does not al-
ways allow us to recognize biologically mean-
ingful entities.
There are many similar examples of over-
classification. Castillo and Spooner (1997) sug-
gested a reduction in series
Conicibaccata
to only
16
(or even 8) taxa, instead of the
40
species
of Hawkes (1990). Van den Berg
et al
. (2001)
reduced the four species in the small series
Cir-
caeifolia
to only one species. The difficulty in
separating extremely similar species and in produ-
cing and using identification keys shows the need
for a drastic reduction of the total number of spe-
cies recognized in section
Petota
. Hawkes (1990)
admitted that identification keys were difficult to
construct and use, and added: “I included them
[in his various revisions]. Having had no feed-
back from users I can only conclude that these
keys were either satisfactory or that no one had
the courage to use them!” The multitude of very
similar wild potato species has bewildered users,
and a reduction of the number of species, as, for
example, advocated by the Solanaceae Source
website, will indeed be welcomed.
Infraspecific taxa
In a situation where the delimitation of species is
extremely difficult to outright impossible (e.g. within
the
brevicaule
complex), it is not a good idea—to
put it mildly—to recognize infraspecific taxa.
Jacobs
et al
. (2011) concluded that most of the
subspecies and varieties in this section did not
receive support, with the exception of
Solanum
commersonii
subsp.
commersonii
and
S. commer-
sonii
subsp.
malmeanum
(which have now been