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Screening Hybrid Populations for Nucellar Embryony






When the rootstock breeder selects parents for hybridization, if the rootstock cultivar will be propagated as seedlings, then the parents must be selected to produce nucel- lar progeny. The frequency of progeny that produce mostly nucellar seedlings will be higher if both parents have nucellar embry- ony, but this cross confi guration will pro- duce mostly progeny identical to the


 

mother. This dilemma is unresolved, although screening for DNA markers that predict nucellar embryony would greatly reduce the cost of propagating hybrids that lack the trait. The appropriate decision is determined by the importance, genetic con- trol and ease of selection for the other traits that can be contributed by each potential parent.

Screening hybrids for adequate levels of nucellar embryony can be done in sev- eral ways. The fi rst question is ‘what is an unacceptable number of zygotic (sexual) seedlings? ’ Among existing commercial rootstocks, the proportion of zygotic seedlings ranges from less than 1% up to 50% (Table 5.2). The acceptable proportion will depend on a number of factors. One issue is the degree of diffi culty nursery per- sonnel would encounter in rogueing the seedling population (eliminating off-types). This will depend on the level of heterozy- gosity of the population for easily recog- nized morphological traits such as the trifoliate leaf morphology present in most trifoliate ´ Citrus hybrids. If the parents are fairly closely related (e.g. two mandarins), then it will be more diffi cult to identify the zygotic off-types than if the parents are dis- tantly related. On the other hand, if the par- ents are closely related, the performance of a tree budded on a zygotic seedling may be


 

 
 

Table 5.2. Percentage of polyembryonic and nucellar seedlings in major citrus groups.

Seed parent Seedlings/seed % nucellar

Lemon: Eureka, Lisbon, etc. 1.05–1.06 32–33

Rough lemon 1.24–1.96 54–98

Mexican Lime 1.29 78

Mandarin: Dancy, Kara 1.37–1.71 100

Mandarin: Satsuma 1.44 90

Mandarin: Kishiu 1.00 0

Mandarins: King, Ponkan 1.01-1.42 21–98

Grapefruit: Marsh 1.08 96

Pummelo: 11 cultivars 1.00 0

Sweet orange: four cultivars 1.09–2.00 39–97

Sour orange 1.21 85

Tangelo: Orlando, Minneola 1.31–1.49 83–97

Trifoliate orange 1.03–1.26 13–73

Source: after Frost and Soost (1968).


 

 

fairly similar to that of a tree budded on a nucellar seedling. Evidence (Xiang and Roose, 1988) suggests that, at least in some locations, most zygotic seedlings result from selfi ng rather than outcrossing. This makes zygotic seedlings more diffi cult to identify, but also makes their characteris- tics more predictable. Nurseries dislike using rootstocks that are excessively vari- able, so we suggest that 10–15% zygotic seedlings is the maximum permissible level in most cases.

In some rootstocks, particularly Citrus

´ Poncirus hybrids, the proportion of nucel- lar seedlings can be estimated fairly well from a visual screen of 100 or more seedlings. Each seedling should be care- fully examined for leaf morphology, intern- ode spacing, thorn length, stem morphology and any other visible traits (Fig. 5.3). A good practice is to compare the new hybrid with an established rootstock, such as Troyer or Carrizo, which produces a high proportion of nucellar seedlings. Another protocol would be estimate the


 

 

proportion of polyembryonic seeds from a germination test or dissection of mature seeds, but this indirect method is less desir- able because the correlation between the proportion of polyembryonic seeds and the proportion of nucellar seedlings is not well established in most populations. Before release of a rootstock or scion variety, the proportion of nucellar seedlings can be esti- mated more precisely by ‘fi ngerprinting’ a large (100–200) sample of seedlings with many heterozygous DNA markers (unlinked simple sequence repeats (SSRs), amplifi ed fragment length polymorphisms (AFLPs) or inter-simple sequence repeats (ISSRs) are appropriate). Many loci that are heterozy- gous in the female parent should be screened because, for each heterozygous locus, the probability of a selfed seedling having the same genotype as the mother is one in two. Testing fi ve unlinked, co-domi- nant, heterozygous loci gives an overall probability of misclassifying a hybrid as nucellar of (0.50)5 = 1/32 = 0.031, so that about 3% of actual hybrids would be dis-


 

 

Fig. 5.3. Zygotic (identifi ed by isozyme analysis) and nucellar seedlings of trifoliate orange. Note that some zygotic seedlings are dwarf, but many are not.


 


 

carded. Twelve dominant loci would be needed to achieve the same accuracy. This procedure is too laborious and costly to apply to seedlings from each of a large pop- ulation of hybrids as would occur in the early stages of a breeding programme. Again, easily screened DNA markers for the genes that govern the presence and propor- tion of nucellar embyros would be particu- larly valuable because of the cost of propagating many hybrids to fruiting.

 

 






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