Abstract:
White clover is an allotetraploid legume with a genome size of ca 960 Mb and is
widely used in Irish grasslands to improve the nutritional value of the forage by
increasing the relative amounts of nitrogen present. White clover is closely related to
Medicago truncatula, the diploid (genome size of ca 500 Mb) model legume chosen
to study the symbiotic genetic programmes of plants and facilitate the genetics and
breeding of important legume crops.
This thesis describes the construction of a genetic map of a white clover mapping
population composed of 94 F1 progeny bred in the Institute of Grassland and
Environmental Research (UK) from parents S1S4 and R3R4. The genetic map was
constructed using amplified fragment length polymorphism markers and white clover
microsatellite markers from previously published white clover maps. The thesis also
details a glasshouse and field based phenotypic analysis carried out on morphological
traits which allowed quantitative trait loci (QTL) to be detected on the genetic
linkage map of white clover. Due to the high correlation between the traits measured,
both conservation of QTLs across environments and co-localisation of QTLs for
related traits were observed in this study.
A BAC library of one of the family parents, R3R4, was constructed using pIndigo
BAC-5, consisting of over 37,000 clones with an average insert size of 85 Kb,
representing a threefold genome coverage. A preliminary sequence analysis was
carried out where 700 BAC clones were sequenced on both ends. Results showed
that over 50% of the BAC-ends had a correspondence with the Medicago genome
sequence; however only 16 of the BAC-end sequence pairs had homology within a
span of 20 to 200 Kb in M. truncatula. The level of synteny between white clover
and M. truncatula was assessed by comparative structural genomics, where five BAC
clones of the white clover library were sequenced to sixfold coverage and their
sequences were compared to the corresponding M. truncatula sequences.
