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Fig. 6. HA6. The order
of A1591, G1016, G1125, A1720, G1130, G1174, pAR49, and
pAR8 on D13 is inverted relative to A10.
Fig. 7. This figure includes two HAs (HA7A and HA7B) joined by a reciprocal translocation between A
t
LGs Chr. 5A and Chr. 4A. A
heavily dotted line separates HA7A from HA7B. A lightly dotted line connects two loci whose putative homoeology may contradict
the inferred linkage group homologies depicted (see text). (A) HA7A. The orders of pAR169 and pAR65 on Chr. 20D relative to D9
and A1808, A1650, and pAR278 on D9 relative to A6 are inverted. The locations of G1112 and G1066 on Chr. 20D differ from their
locations on Chr. 5A and A6, respectively. (B) HA7B. The order of G1033 and A1172 on D12 relative to A14 is inverted.
RFLP evidence is unambiguous with respect to the magni-
tude of structural changes: allotetraploidy in Gossypium was
not accompanied by extensive restructuring.
The question naturally arises as to whether this structural
conservation is true for other allopolyploids. At present, this
is a difficult question to address, as few comparative
mapping data exist for diploids and their allotetraploid de-
scendants. Perhaps the best example is Brassica. The three
diploid genomes in B. rapa (A), B. nigra (B), and B. olera-
cea (C) are extensively rearranged relative to each other
(Lagercrantz and Lydiate 1996; Quiros et al. 1994), but the
A and C genomes of the allotetraploid B. napus are highly
conserved relative to B. rapa and B. oleracea (Bohuon et al.
1996; Lydiate et al. 1993; Parkin et al. 1995). Cheung et al.
(1997) inferred that major genome rearrangements differen-
tiate B. napus and B. oleracea, but they did not distinguish
between the A and C genomes in B. napus. Because the A
and C diploid genomes are structurally different, many of
the rearrangements they observed probably reflect differ-
ences between the C genome of B. oleracea and the A ge-
nome of B. napus. Nonreciprocal translocations may arise
from homoeologous recombination in doubled haploids of
Brassica napus (Sharpe et al. 1995), but the relevance of
this observation to rearrangement in natural allopolyploids is
not clear. Similarly, it may be that the unexpected gains
and losses of restriction fragments observed in synthetic al-
lopolyploids (Song et al. 1995) were partly due to genome
rearrangement on the scale detectable by comparative RFLP
mapping, but other mechanisms are perhaps more likely
(Feldman et al. 1997; Liu et al. 1998; Song et al. 1995).
Thus in Brassica, as in Gossypium, there is little direct
evidence for structural rearrangement directly associated
with polyploidy itself (Parkin et al. 1995).
Genome duplication and paleopolyploidy in Gossypium
One of the more notable features of genetic maps of dip-
loid plants is that many genomic regions appear to be dupli-
cated. For example, approximately 50% of loci in the
Brassica A and C genomes are duplicated (Quiros et al.
1994), and in Sorghum 41% of probes detected more than
one restriction fragment (Pereira et al. 1994). While there
are a number of mechanisms by which loci may become du-
plicated, the occurrence of conserved linkage blocks shared
by two chromosomes within a diploid genome or two chro-
mosomes within a polyploid genome has been taken as evi-
dence of paleopolyploidy (e.g., Chittenden et al. 1994;
Helentjaris et al. 1988; Kianian and Quiros 1992; Kowalski
et al. 1994; Whitkus et al. 1992). Given an evolutionary his-
tory of repeated cycles of polyploidization in angiosperms,
ancient duplicated linkage blocks should be detectable in
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Probes revealing
two loci within a
linkage group
Map location
Map location of
homoeologs
A genome
G1130
A10
D13
(Fig. 6)
D genome
A1124
D6
A12
(Fig. 3)
A1159
D12
A14, Chr. 5A
(Fig. 7B)
A1590
D4
A16, LG D03, LG A02
(Fig. 4)
G1070
D9
(Fig. 7A)
pAR145
D11
(Fig. 10)
pAR163
D7
LG D02
(Fig. 5)
pAR168
D1
A4, LG A05, LG D01
(Fig. 1)
pAR173
D1
A4, LG A05, LG D01
(Fig. 1)
pAR202
D9
(Fig. 7A)
AD genome
pAR173
LG D01
D1, A4, LG A05
(Fig. 1)
pAR183
LG D(?)05
(Fig. 2)
A1183
Chr. 10A
(Fig. 10)
Table 3. Intra-linkage group duplications in the A, D, and AD genetic linkage maps. Probes revealing loci
located within or near putative rearrangements are denoted in boldface.
Fig. 8. This figure includes three HAs (HA8A, HA8B, and HA8C) united by a reciprocal translocation involving A
t
linkage groups
Chr. 2A, and LG A06 and a confounded A linkage group (A5) that arises because the A genome diploid F
1
parent of the F
2
progeny
is heterozygous for a reciprocal translocation. A heavily dotted line separates HA8A from HA8B from HA8C. Lightly dotted lines
connect loci whose putative homoeology may contradict the inferred linkage group homoeologies depicted (see text). (A) HA8A. The
order of G1164, A1418, and PXP2–60 on LG A06 is inverted relative to D5. (B) HA8B. Some of the HA8B D
t
, D, and A
t
loci have
putative counterparts mapping to the confounded A genome linkage group, A5. Interspersed among these loci on A5 are loci with
homologues mapping to linkage groups in HA8C which otherwise comprise one A
t
(Chr. 1A), one D (D2), and one D
t
(Chr. 15D)
linkage groups. Within HA8B, the order of pAR185, pAR149, and pAR172 on LG A06 is inverted relative to D3 and Chr. 17D.
(C) Within HA8C, the order of A1553, A1225, pAR11, P1–3, pAR88, and A1720 on D2 is inverted relative to Chr. 15D, and the
locations of A1097 and A1794 on Chr. 1A differ from their positions on D2.