153its mode of action have been disclosed (e.g., Liu et al. 2006 ; Nicolas et al. 2009 ;
Cifuentes et al. 2010 ; Grandont et al. 2014 ). A main difference relative to the wheat
Ph1 locus is that allelic variation for high and low homoeologous pairing at the
PrBn locus greatly infl uences the pairing behavior of haploids but has no effect in
some interspecifi c hybrid com binations (e.g., Lefl on et al. 2006 ).
Jenczewski and Alix ( 2004 ) documented other important allopolyploid crops,
like coffea ( Coffea arabica ; 2 n = 4× = 44, CCEE) or tobacco ( Nicotiana tabacum ;
2 n = 4× = 48, SSTT), for which indirect evidences suggest that their diploid-like
meiotic behavior may be governed by Ph -like genes. Sorghum ( Sorghum bicolor ;
2 n = 4× = 20, AABB) can be added to the list after the demonstration of MI pairing
between its constituent genomes in interspecifi c hybrids with the related po lyploid
S. macrospermum (Kuhlman et al. 2008 ).
The potential application to Arabidopsis suecica (2 n = 4× = 26) of all analytical
tools developed in the model plant Arabidopsis explains the recent research interest
in this allopolyploid species (Bomblies and Madlung 2014 ). A. suecica , which orig-
inated from the hybridization between the diploids A. thaliana (2 n = 2× = 10) and A.
arenosa (2 n = 2× = 16), shows a strict bivalent pairing behavior at metaphase I
(Comai et al. 2003 ). Henry et al. ( 2014 ) have conducted a comparative analysis
between A. suecica and synthetic amphiploids carrying A. thaliana and A.
arenosa genomes which supports that the higher meiotic stability in the natural
allotetraploid is not related to structural chromosomal differentiation between its
constituent genomes but to some Ph -like mechanism that inhibits homoeologous
recombination. These authors have further detected a locus on the A. arenosa
genome whose A. suecica allele seems to suppress homoeologous synap sis an d
therefore multivalent formation.
6.8 Perspectives
The outstanding relevance of common and durum wheat as a crop in addition to the
great list of chromosome variants stock lines have facilitated more advances in the
knowledge of the control of meiosis in wheat than in other polyploid species.
However, approaches carried out are not yet conclusive on the functional nature of
Ph genes. Though some studies of meiosis in wheat published in the last 15 years
have contributed to the general knowledge of the meiotic process in plants, most of
the recent advances in this fi eld have been obtained in diploid species, specially in
Arabidopsis. Publication of the draft sequence of the bread wheat genome is
expected to generate genomic tools that ensure the identifi cation of genes such as
Ph1 or Ph2. This seems essential to decipher their mode of action. Also of capital
interest is the identifi cation of Ph I^ genes as well as the molecular mechanism
whereby they interact with Ph genes, which may open new ways of manipulating
homoeologous pairing. The most relevant feature of the Ph and Ph I^ genes, their abil-
ity to induce wheat– alien interspecifi c gene transfer, maintains still the potential to
be applied in future breeding related programs.
6 The Mode and Regulation of Chromosome Pairing...