The genetic stability of a recombinant form of the Lr19 translocation of wheat

Vermaak, Charmaine (1999-12)

Thesis (M.Sc. Agric.) -- University of Stellenbosch, 1999.

Thesis

ENGLISH ABSTRACT: The Lr19 gene is an excellent source of leaf rust resistance worldwide. It occurs on a translocated segment on chromosome 7DL in Triticum aestivum and was derived from Thinopyrum ponticum. Known genes on the translocation are: centromere - Sd1 (segregation distortion) - Xpsr165 - Xpsr 105 - Xpsr129 - XcsIH81-1 - Lr19 - WSP-D1 (water soluble protein) - Sr25/Y (stem rust resistance/ yellow endosperm). Following the disruption of meiotic pairing behaviour in a Lr19 heterozygote, a recombinant, Lr19 (149), was selected (Marais, 1992c). In the recombination event Lr19 (149) was relocated to chromosome arm 7BL with wheat chromatin on both sides of the translocation. Lr 19 (149) has lost Y1, Sr 25 and Sd1. In translocation heterozygotes, gametes with Lr19 (149) have a strong tendency to self eliminate. The purpose of this study was (1) to determine if self-elimination occurs in heterozygotes of both sexes, (2) if the Lr19 (149) translocation can recombine with homoeologous regions on 7BL in the presence of PhI (pairing inhibitor) gene, (3) to determine whether the selfelimination tendency of the translocation is accompanied by an increased incidence of mutations. Strong self-elimination of Lr19 was detected in F2 and F3 populations. The self-elimination, which is influenced by the genetic background, was found to be more pronounced when the segment was transmitted via pollen. No recombination was detected between Lr19 and two proximally located loci: Xus-OPK91350 and XcsIH81-1, and also not between two distally located loci: Wsp-D1c and X12c. This suggests that the translocation is transmitted as a single, large linkage block during meiosis. The reason for this is probably the PhI gene which regulates homology recognition along the entire length of the chromosome. No mutations were found at the four marker loci. Thus, if Lr19 (149) is used in breeding, its transmission will be impaired on the segregating generations and the selection of superior Lr19 (149) homozygotes will be complicated. Fortunately, this will not be accompanied by an increased tendency for mutation. An attempt to convert the csIH81-1 probe into a STS (sequence-tagged-site) marker was not succesful as no useful polymorphisms could be obtained, even after using different enzymes to cut the amplification product.

AFRIKAANSE OPSOMMING: Die Lr 19 geen is wereldwyd 'n uitstekende bron van blaarroes-weerstand. Dit kom voor op 'n translokasie op chromosoom 7BL van Triticum aestivum en is verhaal van Thinopyrum ponticum. Bekende gene op die translokasie is: sentromeer - Sd1 (segregasie distorsie) - Xpsr 165 - Xpsr 105 - Xpsr 129 - XcsIH81-1 - Lr 19 - WSP-D1 (water oplosbare proteien) - Sr25/Y (stamroes-weerstand/ geel endosperm). Nadat die meiotiese paring in 'n Lr 19 heterosigoot ontwrig is, is 'n rekombinant, Lr19 (149), geselekteer (Marais 1992c). Tydens rekombinasie is Lr19 (149) verplaas na chromosoom-arm 7BL in 'n dubbel-oorkruisings-gebeurtenis. In die proses is Thinopyrum chromatien verruil vir koring-chromatien aan beide kante van Lr 19. Lr19 (149) het Y1, Sr25 en Sd1 verloor. Gamete met Lr19 (149) toon 'n sterk neiging om te self-elimineer in translokasie heterosigote. Die doel van hierdie studie was om te bepaal of: (1) self-eliminasie van gamete in beide geslagte van heterosigote plaasvind, (2) die Lr19 (149) translokasie, in die teenwoordigheid van die Phi (parings inhibeerder) geen, kan rekombineer met homoeoloe streke op chromosoom-arm 7BL, en (3) die translokasie se self-eliminerings-neiging gepaard gaan met 'n verhoogde voorkoms van mutasies. Sterk self-eliminasie van Lr 19 in heterosigote is waargeneem in F2 en F3 populasies., Die self-eliminasie effek was sterker ten opsigte van oordrag deur die stuifmeel. Geen rekombinasie is tussen Lr19 en twee proksimaal gelee loki, Xus-OPK91350 en XcsIH81- 1 waargeneem nie, en ook nie tussen Lr19 en twee distaal gelee loki, Wsp-D 1c en X12c, nie. Hieruit kan afgelei word dat die translokasie gedurende meiose oorgeerf word as 'n enkel, groot koppelingsblok. Die rede hiervoor kan wees dat die homologie-vlak oor die hele chromosoom op molekulere vlak getoets word voordat sinapse en oorkruisings plaasvind gedurende meiose. Daar word geglo dat die Phl geen sodanige homologie-herkenning reguleer. Geen mutasies is by die vier merker loki gevind nie. Dus, indien Lr19 (149) in teling gebruik word, sal dit noodgedwonge as 'n koppelingsblok oorerf 'en seleksie vir verbeterde Lr19 (149) homosigote sal oneffektief wees. 'n Voordeel is dat dit nie met 'n verhoogde mutasie vermoe gepaard sal gaan nie. 'n Poging wat aangewend is om die csIH81-1 peiler om te skakel na 'n STS ("sequencetagged- site") merker was nie suksesvol nie omdat geen bruikbare polimorfisme verkry is nie. Selfs nadat verskillende restriksie-ensieme gebruik is om die amplifikasie-produk te sny, is daar steeds geen nuttige polimorfismes waargeneem nie.

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