Marker-Assisted Selection for Biotic Stress Resistance in Peanut - Translational Genomics for Crop Breeding: Biotic Stress - page 128

species have been described as A. decora , A.
palustris , and A. praecox (Lavia 1996, 1998;
Penaloza and Valls 2005; Valls and Simpson
2005).

the Americas to other parts of the world since
the 16th century. Wynne and Coffelt (1982)
indicated the existence of an important sec-
ondary center of diversity within A. hypogaea
in Africa, where a large amount of variation is
thought to arise from hybridization and selection
in different environments.
Krapovickas and Gregory (1994) classified A.
hypogaea into two subspecies and six botani-
cal varieties. A. hypogaea subsp. hypogaea is
characterized by a spreading growth habit, alter-
nating vegetative and reproductive nodes, lack
of flowers on the mainstem, medium-to-large
seeds, medium-to-late maturity, and includes
the botanical varieties hypogaea (virginia and
runner market types) and the less frequently
cultivated hirsuta . Several genotypes cultivated
among native Brazilian Indians from the Xingu
region have been characterized morphologically
and using molecular markers and are reported
to group with the hypogaea subspecies (Freitas
et al. 2007). The fastigiata subspecies is typ-
ified by erect growth habit, sequential repro-
ductive nodes, flowers on the mainstem, small
seeds, and early maturity. These include the
botanical varieties fastigiata (valencia), vulgaris
(spanish), peruviana , and aequatoriana . The lat-
ter two are not cultivated widely outside of
Peru, northwestern Brazil, northern Bolivia, and
Ecuador.

Origin of Arachis Hypogaea

A. hypogaea is a tetraploid (2n 40) (Husted

1936), and the only other known tetraploid
species in the section, A. monticola , is closely
related to it. Hybridization between the culti-
gen and section Arachis diploids is possible,
but no evidence has been found that this has
contributed to ongoing gene flow into the culti-
gen in nature. Cultivated peanut is considered to
be an AB tetraploid, arising from hybridization
between A and B diploid species (Smartt et al.
1978).
Lack of marker polymorphism in the culti-
gen using RFLP and RAPD markers (Halward
et al. 1991; Kochert et al. 1991) contributed to the
hypothesis that all varieties and botanical types
of A. hypogaea share common diploid progeni-
tors (Kochert et al. 1996). RFLP analysis deter-
mined that A. duranensis had greater similarity
to A. hypogaea than did A. cardenasii (Kochert
et al. 1991, 1996), and A. duranensis is consid-
ered by many now to be the likeliest A-genome
ancestor. However, subsequent marker analyses
have also proposed A. villosa (Raina and Mukai
1999), A. helodes , and A. simpsonii (Milla et al.
2005b) as potential A-genome donors.
Evidence from archaeological data (Simp-
son and Faries 2001), molecular marker data
(Kochert et al. 1991, 1996), fluorescent in situ
hybridization analysis using rDNA as labeled
probe (Raina and Mukai 1999; Seijo et al.
2004), and gene sequence data (Jung et al.
2003; Ramos et al. 2006) strongly supported A.
ipaensis instead of A. batizocoi as B genome
donor. However, new discoveries of wild Arachis
species are still being made (Valls and Simpson
2005), and it is possible that other candidates
could be discovered.
As a result of explorations by the Span-
ish and Portuguese, peanut spread quickly from

= 4x =

Introgression Pathways

Attempts to utilize wild species as sources of
new alleles have been met with limited success
because of genomic (A and B genomes) and
ploidy (diploid and tetraploid) barriers (Stalker
and Moss 1987). Several pathways have been
attempted with varying degrees of success, of
which this chapter covers three: the hexaploid
route, the autotetraploid route, and the allote-
traploid route (the latter more commonly known
simply as the tetraploid route).
The hexaploid route involves crossing a
diploid wild species with A. hypogaea to gener-
ate a sterile triploid hybrid, followed by doubling