Agriculture Reference
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Heslop-Harrison 1990; Song et al. 1995; Riera-Lizarazu et al. 1996;
Feldman et al. 1997; Wendel and Crone 2003), but often result in sterile
offspring. The most common point of failure of progeny of such crosses is
embryo abortion (Muller 1925; Mable 2004). That this abortion is
frequently associated with miss-paired genomes during meiosis can
be seen from the ability to restore fertility by chemically induced genome
doubling (Fasoulas 1993) where colchicine-induced replicated, but not
the homologous genomes pair during meiosis, resulting in essentially no
genetic recombination in F2 generation populations. Negative effects of
loss of recombination in polyploids appear to be outweighed by the
positive effects of stable heterosis (Hilu 1993; Arnold 2004; Herr et al.
2005) and other mechanisms that enhance variations in gene expression
and their subsequent positive phenotypic effects (McClintock 1984;
Arnold 2004; Jorgensen 2004; Fontdevila 2005; Chen 2007).
The inability of a large portion of the genome to pair in wide crosses
duringmeiosis when paternal andmaternal DNA strands come together
at the Double Holliday Junctions, which are created by the invasive
strand mechanism, often results in a failure of embryo development
(Petronczki et al. 2003). Loss of the critical initiation of this pairing
process in Spo11 yeast mutants is lethal because dramatically un-
balanced recombination and subsequent aneuploidy occurs (Cao et al.
1990, 2000; Dernburg et al. 1998). Thus, although zygote lethality asso-
ciated with impaired meiotic paring also can be the result of highly
frequent unbalanced crossover events that allow the loss of essential
genetic loci during the formation of the haploid gametes, this likely
occurs only in the most diverse crosses. Meiotic pairing events do not
occur at completely random sequences (Baudot and Nicolas 1997). One
would thus expect some conservation of the preferred regions, which
apparently are concentrated in promoter and amino acid coding (chro-
matin accessible) regions, and crossover events at some regions prevent
crossovers at nearby regions (Muller 1916; Blat et al. 2002; Shapiro and
VonSternberg 2005). However, wide crossesmay just not allowsuccess-
ful pairing at chiasmata of paternal andmaternal genomes that have very
different organizational structures, for example, distance between and
speci
city of intra- and intergenic regions, which can often result from
rapid ampli
cation of transposons (Shapiro and Von Sternberg 2005).
Another intriguing possibility is that a failure of mutual compliance of
widely different nuclei during hybridization may not allow proper
epigenetic reprogramming of genomes that is required for ontogeny,
as has been observed in animal nuclear heterokaryon experiments (Blau
et al. 1983). Failure of diverse hybridizations may also occur because of
