Genetic control of Fusarium circinatum tolerance in Pinus patula x P. tecunumanii hybrid families

Mphahlele, Lebogang Ivonne (2020-04)

Thesis (MScFor)--Stellenbosch University, 2020.

Thesis

ENGLISH ABSTRACT: This study concentrated on the genetic control of Fusarium circinatum in Pinus patula × P. tecunumannii high elevation (HE) and P. patula × P. tecunumannii low elevation (LE) hybrid families. A greenhouse method of artificial inoculation screening was employed. The genetic material used included 37 P. patula × P. tecunumanni LE and 32 P. patula × P. tecunumannii HE hybrid families together with their parental families of two P. patula, four P. tecunumannii LE, five P. tecunumannii HE and four P. taeda. Greenhouse screening were performed with the inoculation of three cultures simultaneously onto the abscission growth tip of seedlings. After eight weeks, plant height and lesion length were measured to calculate stem-kill percentage as an indicator of F. circinatum tolerance. The smaller the stem-kill percentage, the higher the F. circinatum tolerance. A strong positive correlation (93.6%) was observed between stem-kill percentage and lesion length, while a weak negative correlation (40.6%) was observed between stem-kill percentage and plant length. Genetic parameters such as narrow sense heritability (h2), general combining ability (GCA) and specific combining ability (SCA) was employed to determine which parental family contributed the most to F. circinatum tolerance. Analyses were conducted on two categories. The first one combined both hybrid families (combined dataset for PPTH and PPTL) and the second one separated hybrid families (PPTH separated from PPTL). The assumption was PPTL might mask PPTH hybrid families. Least square means were also calculated to rank hybrid families and species in terms of F. circinatum tolerance. Significant differences (p < 0.0001) were obtained for hybrid families and species tested for all genetic parameters tested. Variance components estimates indicated that the male variance components (o2m)of P. tecunumanii HE and P. tecunumanii LE contributed more to F. circinatum tolerance with a gene frequency of 4.30%, while the female variance components (o2f) indicated that P. patula, as female parent, contributed less with a 1.60% gene frequency. Obtained narrow sense heritability h2 also indicated that h2 male had a strong genetic control for both male parents, while the h2 female indicated a low genetic control. This was confirmed with the GCA obtained by most of the male and female parents. The parental family (TL2) from P. tecunumanii LE population had a negative and low GCA (-11.42), indicating more genetic effect contribution to P. patula and thus, a high level of F. circinatum tolerance. Hybrid family P5 × TL1 achieved a low and negative SCA (-20.02), indicating a high level of tolerance thus, additive and non-additive interaction between genes influenced the phenotype of hybrids. As a novelty, this study’s results were compared to the frost tolerance of the same genetic material of a previous study by Malinga (2019). Although a negative correlation in general was observed between frost and F. circinatum tolerance, two hybrid families indicated stronger F. circinatum and frost tolerance. Therefore, breeders should consider crossing P. patula with P. tecunumanii (LE and HE) families screened in this study based on the GCA and SCA values obtained. However, the combination of frost and F. circinatum resistant commercial hybrids might be more difficult to obtain.

AFRIKAANSE OPSOMMING: Hierdie studie het gefokus op die genetiese beheer van Fusarium circinatum in Pinus patula × P. tecunumannii hibried families. ‘n Kwekery metode van kunsmatige inokulasie was toegepas. Die volgende genetiese material is gebruik: 37 P. patula × P. tecunumanni LE en 32 P. patula × P. tecunumannii HE hibried families, asook die ouers wat twee P. patula, vier P. tecunumannii LE, vyf P. tecunumannii HE en vier P. taeda families ingesluit het. Kwekery toetse was uitgevoer deur ‘n mengsel van drie kulture te inokuleer op die aktiewe groeipunt van saailinge. Na agt weke is die plant lengte en letsel lengte gemeet en stam-dood persentasie bereken as ‘n aanduiding van F. circinatum verdraagsaamheid. Hoe kleiner die stam-dood persentasie, hoe hoër is die F. circinatum verdraagsaamheid. ‘n Sterk positiewe korrelasie (93.6%) was waargeneem tussen stam-dood persentasie en letsel lengte, maar ‘n swak negatiewe korrelasie (40.6%) was waargeneem tussen stam-dood persentasie en plant lengte. Genetiese aanwysings (parameters), byvoorbeeld eng oorerflikheid h2, algemene kombineer vermoë (GCA) en spesifieke kombineer vermoë (SCA), was bereken om te bepaal watter ouerfamilie die meeste tot F. circinatum verdraagsaamheid bydra. Ontledings was uitgevoer in twee stappe: gekombineerde hibried families (kombineer die PPTH en PPTL datastelle) asook afsondelike hibried families (PPTH afsonderlik van PPTL). Dit was gedoen weens die aanname dat PPTL dalk die effek van PPTH hibried families kan oorheers. Families is verder gesorteer volgens F. circinatum verdraagsaamheid. Betekenisvolle verskille (p < 0.0001) was waargeneem vir hibried families en spesies vir al die getoetse genetiese aanwysings. Variansie komponente het aangedui dat die manlike variansie component o2m van P. tecunumanii HE en P. tecunumanii LE meer bygedra het tot F. circinatum verdraagsaamheid met ‘n geen frekwensie van 4.30%. Die vroulike variansie component o2f het aangedui dat P. patula, as vroulike ouer, minder bygedra het met ‘n geen frekwensie van 1.60%. Waargenome eng oorerflikheid h2 het ook aangedui dat h2 male ‘n sterk genetiese beheer oor beide manlike ouers het, terwyl h2 female ‘n lae genetiese beheer aangedui het. Dit was bevestig met die GCA van beide manlike en vroulike ouers. Die ouerfamilie (TL2) van P. tecunumanii LE populasie het ‘n negatiewe en lae GCA (-11.42) gehad wat gedui het op ‘n sterker genetiese effek as P. patula, en dus ‘n hoë vlak van F. circinatum verdraagsaamheid. Hibried familie P5 × TL1 het ‘n lae en negatiewe SCA (-20.02) gehad wat gedui het op ‘n hoër vlak van verdraagsaamehid, en dus het toegevoegde en nie-toegevoegde interaksie tussen gene die fenotipe van hibriede beïnvloed. Uniekheid van die studie was die bepaling van ‘n moontlike korrelasie tussen koue en F. circinatum verdraagsaamheid van dieslefde genetiese materiaal soos bepaal deur Malinga (2018). Alhoewel ‘n negatiewe korrelasie waargeneem is, het twee hibried families ‘n sterk moontlikheid getoon vir beide koue en F. circinatum verdraagsaamheid. Daarom moet telers oorweeg om P. patula en P. tecunumanii (LE and HE) families te toets gebasseer op die GCA en SCA waardes wat in die studie waargeneem is. Die moontlikheid om ‘n hibried te kry met beide koue en F. circinatum verdraagsaamheid mag dalk meer kompleks wees.

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