Agriculture Reference
In-Depth Information
was criticized by Correll (1962) in his mono-
graph of the group, where he himself recog-
nized a total of
191
species within subsection
Hyperbasarthrum
(a name pointing at a common
characteristic of the tuber-bearing species, the
presence of an articulation above the base of the
pedicel). Hawkes followed his lead by publishing
a series of treatments (Hawkes, 1956, 1963,
1990), finally recognizing
232
species, including
7
cultivated species, within the group that was
now called section
Petota
Dumort.
Hawkes and his Peruvian colleague, Ochoa,
proceeded to dominate the field for decades, pro-
ducing a number of landmark studies (Hawkes
and Hjerting, 1969, 1989; Hawkes, 1990; Ochoa,
1990, 2004). Interestingly, Hawkes and Hjert-
ing (1989) and Ochoa (1990) focused on the
same material, the wild species of Bolivia, enab-
ling a comparison between their approaches.
Spooner and Van den Berg (1992a) highlighted
the apparent difficulty these experts experienced
in consistently identifying herbarium specimens,
especially when dealing with taxa that were
closely related to the cultivated potato. Spooner
and collaborators contributed a number of de-
tailed studies of parts of section
Petota
(e.g. Spooner
and Van den Berg, 1992b; Giannattasio and
Spooner, 1994a,b; Spooner
et al
., 1997; Van den
Berg
et al
., 1998, 2002; Miller and Spooner,
1999), criticizing the use of a “splitter” approach
and a typological species concept.
Still, Hawkes's 1990 treatment became the
standard one, used extensively by gene banks and
potato breeders. The topic gives keys to and a short
description of all recognized species, and classifies
them in
21
series that are arranged in two “su-
perseries” based on the outline of the flowers (stel-
late or rotate). It recognizes many hybrids.
Since 1990, a wide range of molecular
markers has been used for testing the validity
and interrelationships of potato species. This
has resulted in a gradual reduction of the num-
ber of species recognized. Spooner and Hijmans
(2001) reduced the number of species to
206,
and Spooner and Salas (2006) recognized
188
wild and one cultivated species. More recently,
Spooner and co-workers have contributed a
final revision to the Solanaceae Source website
(
http://solanaceaesource.org/
), with
166
ac-
cepted taxa (
156
species).
The group of wild relatives of the cultivated
potato displays a polyploid series from diploid to
hexaploid. Since these polyploids are interpreted
as allopolyploids, progenitor-derivative rela-
tionships can be hypothesized, and these can be
expressed using genome formulae (Matsubayas-
hi, 1991). The diploid species occurring in South
America share a common genome that is also
present in
Solanum verrucosum
, a species occur-
ring in Mexico. This common genome is indicated
with AA by Matsubayashi (1991). The polyploid
species present in Central America and Mexico
resulted from crosses, and the contributions of
the parental species can be expressed in the gen-
ome formula of the polyploids, e.g. AABB for
tetraploid species of the Mexican series
Longi-
pedicellata
and AADDD
d
D
d
for the hexaploid
Solanum demissum
. In this way, evidence could be
provided for the evolutionary relationships between
the diploid and polyploid species. In addition to
this, based on their crossability and the ratio of
maternal and paternal genomes, an endosperm
balance number (
1,
2,
4) has been assigned to the
species, indicating their membership to crossa-
bility groups (Johnston and Hanneman, 1978).
Summarizing the results of the quite exten-
sive phylogenetic studies that have been performed,
a rather robust phylogeny reconstruction is
apparent: almost all of the produced trees have
a similar topology with the following order of
appearance of groups within section
Petota
:
•
1
endosperm balance number (EBN)
Mexican diploids
•
1
EBN South American diploids
•
2
EBN South American diploids
•
2
EBN South American polyploids
•
4
EBN Mexican polyploids
This conforms with Hawkes's (1990) scen-
ario of an origin in Central America, migration
to South America, followed by diversification in
the microhabitats provided in the Andes moun-
tain range and a re-migration towards the north,
accompanied by polyploidization. However, the
data of Volkov
et al
. (2003), for example, did not
support this scenario. These authors, working
with rDNA external transcribed spacers concluded
that all “primitive” representatives of section
Petota
originated in South America, and that later
primitive species with stellate flowers migrated
to Central America and Mexico. Also, the high
similarity of resistance (R) genes found in
Solanum
bulbocastanum
and
Solanum stoloniferum
strongly
suggested that the diploid species
S. bulbocastanum
