152
s trict homologous chromosome pairing is believed to be under genetic control.
A brief summary of mostly old but some recent evidences of Ph -like systems fi ts the
scope of this chapter, most examples being thoroughly reviewed by Jenczewski and
Alix ( 2004 ).
Cultivated oats ( Avena sativa and A. bizantina ) are allohexaploid species with
exclusive homologous bivalent formation at meiosis. The existence of a diploidiz-
ing genetic control system in hexaploid oat was early evidenced by the meiotic
analysis of nulli-polyhaploids of A. sativa (Gauthier and McGinnis 1968 ). The com-
plex mechanism regulating bivalency in oat, which seems to involve several chro-
mosomes with distinct effectiveness (Gauthier and McGinnis 1968 ; Leggett 1977 ),
shows a close similarity with that of wheat (reviewed in Jauhar 1977 ). As occurs in
wheat, the homologous pairing suppression can be disrupted by certain genotypes
of alien species. Rajhathy and Thomas ( 1972 ) reported a line of the diploid relative
A. longiglumis which induces chromosome synapsis in interspecifi c hybrids with A.
sativa. The promoting effect of this A. longiglumis genotype has indeed been suc-
cessfully used for introgressive br eeding of common oat (Thomas et al. 1980 ).
Fescues ( Festuca ssp.) and r yegrasses ( Lolium ssp.) form a singular complex of
species with intergeneric hybrids showing a remarkable high level of homoeologous
MI pairing, though the genomic relationship between parental chromosomes is dis-
tant enough for easy GISH-based cytological discrimination (e.g., Kopecký et al.
2008 ). The diploid-like behavior of the allohexaploid tall fescue ( Festuca arundina-
cea ) and other polyploid Festuca species seems to be controlled by homoeologous
pairing suppressors that, unlike the wheat Ph genes, are ineffe ctive in the h aploid
hemyzygous state (Jauhar 1975a , b ). This has been confi rmed by meiotic analyses
of several Festuca × Lolium intergeneric hybrid combinations where all constituent
genomes pair almost freely (see Kopecký et al. 2009 and references therein).
Genotypes that suppress homoeologous pairing in interspecifi c and intergeneric
hybrids have also been demonstrated in diploid Lolium species, like L. perenne and
L. longifl orum (Taylor and Evans 1977 ; Armstead et al. 1999 ).
The presenc e of a Ph -like genetic control in cultivated cottons Gossypium bar-
badense and G. hirsutum (both being 2 n = 4× = 52, AADD) was inferred from the
observation that their haploids show almost no pairing at metaphase I whereas up to
11 bivalents have been reported in some interspecifi c hybrids between their putative
diploid progenitors (Kimber 1961 ; Mursal and Endrizzi 1976 ). Some authors have
questioned the need of a homoeologous pairing suppressor activity to explain the
meiotic regularity of polyploid cottons. However, it seems the most substantiated
hypothesis (Wendel and Cronn 2003 ; see Jenczewski and Alix 2004 , and references
therein), additionally supported by recent molecular approaches demonstrating
ancient A–D homoeologous exchanges in both species (e.g., Flagel et al. 2012 ).
Brassica nap us (2 n = 4× = 38, AACC) is an allotetr aploid derived from the dip-
loids B. rapa (2 n = 20, AA) and B. oleracea (2 n = 18, CC). Jenczewski et al. ( 2003 )
proposed the presence of a major locus regulating homoeologous pairing, which
was designated as PrBn (for Pairing regulator in B. napus ). This locus, genetically
mapped to a C genome chromosome, is part of a complex system including several
other loci with minor effect (Liu et al. 2006 ; Cifuentes et al. 2010 ). Several clues on
T. Naranjo and E. Benavente