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6. DO ANY SPECIES BOUNDARIES...
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The nature of processes that occur at the early stages of speciation can be understood
by investigating the patterns of non-Mendelian inheritance of molecular markers in linkage
mapping populations. The hybrid progeny may receive more alleles from one parent than
expected under Mendelian rules. For example, segregation ratios in crosses between pearl
millet, Pennisetum glaucum and P. violaceum can be skewed 12:1 in favour of P. violaceum
alleles (Liu et al. 1996). In crosses between Iris fulva and I. brevicaulis significant ratio
distortion has been detected in spite of the fact that F 1
hybrids are vigorous, highly fertile
and undergo normal meiosis (Bouck et al. 2005). Notwithstanding these striking examples,
a cautionary point related with interfering evolutionary relationships from marker distortions
alone is that the tempo of changes is differing throughout the whole genome. Alleles with
neutral or positive effects will likely segregate according to Mendelian rules even in crosses
between relatively distant species. Despite the fact that hybrids between Lycopersicon esculentum
(tomato) and Solanum lycopersicoides (wild nightshade) display incomplete chromosome
pairing and high pollen sterility, the majority of loci fit Mendelian expectations in the F 2
generation. The segregation distortions, in favour of homozygotes on chromosomes 2 and
5 and heterozygotes on chromosomes 6 and 9 have eventually been revealed by mapping
studies (Chetelat 2000). These studies have also documented severe recombination suppressions,
sometimes along the entire length of chromosomes. Another remarkable discovery
of a unique segregation distortion locus (D) in the cross between Mimulus guttatus and
M. nasutus (monkeyflowers) exemplifies how genetic maps can help to assess the number
and distribution of genetic factors affecting interspecific hybridization. At the most distorted
marker in the D region only 1% of individuals in the F 2
mapping population were M. nasutus
homozygotes. The magnitude of distortions diminished proportionately at increasingly distant
markers (Fishman and Willis 2005). Genetic map based reappraisals have also provided
support for postulated hybrid origin of Helianthus anomalus. Applying 197 mapped molecular
markers, Rieseberg et al. (1995) were able to identify the precise linkage blocks in this
species that had stemmed from the parental species i.e., H. annuus and H. petiolaris. Thus,
genetic maps based on interspecific crosses provide insight into the genetics of divergence
and reproductive barrier formation in incipient species.
Until recently, there are no studies of species boundaries between L. multiflorum and
L. perenne at the level of details provided by a genetic mapping approach. This is partly because
there have been relatively few comprehensive genetic maps for interspecific crosses
(Hayward et al. 1998; Warnke et al. 2004). Two interspecific maps that have already been
published suffer several limitations including the complex composition of mapping populations,
relatively low resolution and high number of unallocated markers. Most mapping studies,
taking at face value the economical importance of ryegrasses, tend to focus on developing
molecular marker systems for L. perenne, which can be used for marker assisted
selection. To address this, the International Lolium Genome Initiative (ILGI) was established
to construct an enhanced genetic map of L. perenne. At present this map contains RFLP
loci detected by heterologous probes from wheat, barley, oat and rice as well as SSR and
AFLP loci (Armstead et al. 2002). It has also been aligned with maps of wheat, barley and
oat, revealing substantial synteny with the genomes of Poaceae (Sim et al. 2005). Notwithstanding
the substantial progress made by ILGI in Lolium genome mapping, this initiative has
hampered further mapping studies based on various mapping populations in different Lolium