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to deal with the man-made variation of a crop
(like cultivar and cultivar group), which could
be termed “culta” instead of taxa (Hetterscheid
and Brandenburg, 1995; Spooner and Hetter-
scheid, 2005).
This sensible approach was also applied by
Huaman and Spooner (2002), who proposed
eight cultivar groups for the cultivated material
of South America. These authors refrained from
completing the classification with a cultivar
group for the modern potatoes, since that assort-
ment should probably be subdivided into several
groups; for example, according to the various
uses to which the material is put. In a “culto-
nomic” classification, there is no need to classify
all of the material, only groups that are deemed
useful should be circumscribed.
An emphasis on ploidy as the criterion to
subdivide the crop has always determined the
classification of cultivated potatoes. Unfortu-
nately, ploidy determinations have not been car-
ried out for most of the accessions present in po-
tato gene banks, and names of cultivated species
have often been applied to this material without
information on chromosome numbers. Ghislain
et al
. (2006) reported that
32
out of
102
acces-
sions of the presumably diploid cultivated spe-
cies
S. phureja
turned out to be either triploid or
tetraploid, and Spooner
et al
. (2007b) found
many exceptions of clustering by ploidy in the
set of potato landraces that they studied with
nuclear SSRs. They concluded that the gene pool
structure of potato landraces should be re-
evaluated, and felt the necessity to reintroduce
the species category for some of the variants of
cultivated potatoes. Within
S. tuberosum
, two
cultivar groups are recognized (Andigenum and
Chilotanum), but the rest of the crop is classified
in the species
S. ajanhuiri
(
2×),
S. juzepczukii
(
3×),
and
S. curtilobum
(
5×).
On the other hand,
the species
S. phureja
was obliterated, because
the material with this name was not consistently
diploid.
While Ghislain
et al
. (2006) still advocated
the use of a cultivar group (rather than a spe-
cies) classification, Spooner
et al
. (2007b) thus
opted for the reclassification of the cultivated
potatoes at the species level. Their NJ tree did
show groups clustering by ploidy, but with
many exceptions and without statistical sup-
port. They propose to recognize the species
S. ajanhuiri
,
S. juzepczukii
, and
S. curtilobum
,
because only that part of the tree shows boot-
strap support values above 50%. However, the
tree presented in Spooner
et al
. (2007b) does
not really show support for these species. There
is 100% support for a branch uniting
S. curtilo-
bum
, and some of the accessions of
S. ajanhuiri
and
S. juzepczukii
, and 89% support for a group
of all other accessions of
S. ajanhuiri
. Thus, it is
unclear why these species should be main-
tained and the other groups (admittedly show-
ing less than perfect clustering by ploidy)
should not. Especially, the decision to remove
S. phureja
is highly debatable. There is a clear
group of material with this name and most of
this material is characterized by the lack of
tuber dormancy. This is exactly a situation
where a cultivar group could be applied suc-
cessfully, according to the definition cited by the
authors: “Cultivar Groups are taxonomic cat-
egories used by the International Code of No-
menclature of Cultivated Plants to associate
cultivated plants with traits that are of use to
agriculturists and are not meant to represent
natural groups or species in any classification
philosophy.” If a certain trait is deemed import-
ant to group material, other characteristics—
like chromosome number—may vary within
that group.
The solution of Spooner
et al
. (2007b)
leaves us with an unfortunate mix of ICBN (Mc-
Neill
et al
., 2006) and ICNCP (Brickell
et al
.,
2009) categories to describe the structure of the
potato crop, while it is highly debatable that the
species level is suitable for cultigens. The draw-
back of this approach is the inherent instability
of a system based on Linnean taxon names that
must conform to the rules of the ICBN, and must
be changed when the rules of the code prescribe
this. Since name changes are undesirable (espe-
cially for cultivated material), a set of rules gov-
erning the conservation of names has been in-
cluded in the ICBN. These allow the continued
use of a name that turns out not to be the oldest
valid name. The inclusion of these rules shows
an awareness that there is a potential problem in
applying ICBN categories to cultivated material.
On the other hand, the ICBN still contains the
Hybrid Appendix that governs the naming of
nothotaxa (taxa resulting from hybridization),
while these hybrids consist of cultivated plants
in most of the cases presented. In all matters
concerning the nomenclature of cultivated
