and homoeologs on linkage groups in other HAs. For the
most part, however, these duplicated loci did not fit the sim-
ple mapping pattern expected for ancient duplicated chromo-
some segments. The 10 nested duplications that fit the
predictions of paleopolyploidy (see “Results”) accounted for
76, or about one third, of the loci under consideration. More-
over, there was no simple pattern of duplicated loci among
the various HAs. It is probable that some duplications were
generated by processes other than paleopolyploidy. Map
densities also may be insufficient to identify ancient, dupli-
cated linkage groups. In addition, it may be that the amount
of time that has elapsed since paleopolyploidization has
been long enough that subsequent mutational change has
disrupted or obscured most of the ancient linkage groups. In
this respect, we note that the entire tribe to which
Gossypium belongs (the Gossypieae) is based on a chromo-
some number of n = 13, implying that paleopolyploidization
antedates the origin of the tribe, which may be 20 to 40 mil-
lion years old (Seelanan et al. 1997, and unpublished data).
inversion footprints?
Twelve probes revealed 13 pairs of loci that mapped to a
single A, D, or AD linkage group (Table 3). Although
intrachromosomal duplication of a locus may not be espe-
cially noteworthy, we draw attention to the correlation be-
tween these duplications and the rearrangements detected.
Ten of the 13 locus pairs are located within or near putative
rearrangements. Of the ten putative rearrangements involv-
ing three or more loci, six include duplicated loci on at least
one linkage group. Only two intra-linkage group duplica-
tions are not associated with putative inversions. This sug-
gests that the duplications may be “footprints” of inversions
and that the process or processes that give rise to inversions
© 1999 NRC Canada
200
Genome Vol. 42, 1999
Fig. 11. Two A (A21, A23), two D (D15, D17), and four allotetraploid (LG U07, Chr. 17D, LG U02, and LG A04) linkage groups
containing loci with orthologues or homoeologues in one of the 13 HAs (they were not incorporated into the HAs because of
contradictory or insufficient evidence). Locations of the putative orthologues and homoeologues are indicated to the right of each
locus.
may not be fully conservative, giving rise to duplications or
deficiencies. Although duplications are evident, identifying
deletions requires finding probes that differentially hybridize
to the genomes being mapped. This possibility was not eval-
uated
because
probes
were
selected
to
maximize
intergenomic comparisons.
Allopolyploidy in Gossypium is associated with
increased recombination
A growing body of evidence indicates that recombination
rates are not correlated with chromosome size. Maize has six
times as much DNA per nucleus as rice and three times as
much
as
sorghum,
yet
all
three
genomes
are
recombinationally similar across conserved linkages (Ahn
and Tanksley 1993; Binelli et al. 1992; Whitkus et al. 1992).
Similarly, Capsicum genomes are four times larger as those
of tomato, yet the two species are recombinationally similar
(Tanksley et al. 1988). Conversely, conserved linkages be-
tween potato and tomato differ in recombinational length by
a factor of 1.4 to 3.6 but are of similar physical sizes
(Bonierbale et al. 1988; Tanksley et al. 1992).
In Gossypium, chromosomes in the A genome diploids are
nearly twice as large as those in the D genome diploids, and
the A
t
and D
t
genome chromosomes retain the sizes of their
diploid antecedents in allotetraploid cells (Endrizzi et al.
1985; Skovsted 1934). These size differences parallel incon-
gruities in nuclear DNA content. The A genome has almost
twice as much DNA per nucleus as does the D genome (2C
= 4 vs. 2 pg; Bennett et al. 1982; Edwards et al. 1974; Ed-
wards and Endrizzi 1975; Kadir 1976; H.J. Price, personal
communication) with near additivity in the allotetraploids
(2C = 5.6–5.8 pg; Gomez et al. 1993; Michaelson et al.
1991; H.J. Price, personal communication). Despite this
nearly two-fold difference in size, recombination in linkage
blocks conserved between the A and D diploid genomes and
between the A
t
and D
t
allotetraploid genomes are essentially
equivalent (5.8% and 4.8%, respectively; Table 2). This re-
sult verifies previous reports of a lack of correlation between
genome size and total recombination in a particularly satis-
fying way, in that the two allotetraploid genomes are in the
same nucleus, thereby controlling for all of the various life-
history, population genetic, and ecological covariables that
might be suspected of effecting recombination rates.
An unexpected result was that although there is no signifi-
cant difference in recombination between genomes that vary
in size by a factor of two, at either the diploid or
allotetraploid level, there was an increase in recombination
in the allotetraploid genomes (Table 2). The D
t
genome was
58.5% larger, recombinationally, than its diploid counterpart,
with a similar increase in recombination in the A
t
genome
(51.5% greater than A). These differences are remarkably
similar to each other, suggesting that the responsible mecha-
nism operates genome-wide in the allotetraploid. Although
these results are based on only a single allotetraploid map-
ping population, Antonio et al. (1996) demonstrated that ge-
netic distance for 300 markers in five different populations
of rice did not vary significantly in any of 12 chromosomes.
Our results should be considered suggestive and in need of
verification with additional crosses, but at present, they sug-
gest that allotetraploidy in Gossypium has been accompanied
by an increased frequency of recombination. Whether this is
true for polyploids in general, relative to their diploid ances-
tors, is worthy of investigation, as is the question of why
this might be the case. At present, a satisfactory explanation
for enhanced recombination in allotetraploids is wanting.
This work was supported by the National Science Founda-
tion (JFW), U.S. Department of Agriculture (AHP), the
Texas Agricultural Experiment Station (AHP), and Texas
Higher Education Coordination Board (AHP).
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