Proc. of Second World Avocado Congress 1992 pp. 545-550

A Study of Avocado Germplasm Resources, 1988-1990. III. Ribosomal DNA Repeat Unit Polymorphism in Avocado

 

G. Bufler

Institut für Obst-, Gemüse- und Weinbau (370), Universität Hohenheim, 7000 Stuttgart 70, Germany

 

A. Ben-Ya'acov

Institute of Horticulture, ARO, The Volcani Center, Bet-Dagan, 50-250, Israel

 

 

Abstract. Nuclear ribosomal DNA of advanced and primitive cultivars of avocado (Persea americana Mill.) collected in Mexico was surveyed for repeat length and restriction site variation. The ribosomal repeat encoding the 18S to 25S ribosomal RNA yields informative fragments after digestion with the restriction enzymes Pstl, Eco Rl and Sstl. Digestion with either Pstl and Sstl or Eco Rl and Sstl are sufficient to identify the botanical variety of an avocado cultivar. Moreover, cultivars representing Persea americana var. guatemalensis Williams and var. americana Mill, reveal a similar fragment pattern with either Pstl or Eco Rl, while the fragment patterns of cultivars of var. drymifolia (Schlecht. and Cham.) Blake are different from the fragment patterns of cultivars of the other two varieties. Assay of 5S ribosomal DNA, which is unlinked to the 18S to 25S ribosomal DNA, support these observations; these preliminary results suggest a closer phylogenetic relationship of var. guatemalensis and var. americana, compared to a more distant lineage of var. drymifolia.

 

 

In a recent review on the origin and taxonomy of avocado (Persea americana Mill.), Scora and Bergh (1990) described P. americana as a polymorphic species consisting of the wild varieties floccosa Mez., steyermarckii Allen, nubigena (Williams) Kopp, and the cultivated varieties americana Mill., drymifolia (Schlecht. and Cham.) Blake and guatemalensis Williams. Furnier et al. (1990) examined the phylogenetic relationships among these taxa by analyzing DNA restriction fragment length polymorphisms (RFLPs) in the chloroplast DNA and nuclear genes coding for cellulase and ribosomal RNA (rDNA). The assay of the variation of rDNA has proven to be a useful tool in plant systematic studies (Hemleben et al., 1988). In the present study we examined the variation of 18S to 25S rDNA and 5S rDNA of advanced and primitive cvs. of avocado collected in Mexico in order to check the identification of the collected material and to gain further insight into the phylogenetic relationships in the subgenus Persea. The results of a selection of representative cvs. are presented.

Materials and Methods

Plant material. Avocado samples collected in Mexico are listed in Table 1. The presumed status of a sample was derived from morphological characters and from the collection source. The tentative identification of the botanical variety in the field was based on morphological characters as described, for example, by Bergh and Ellstrand (1987). Samples of advanced cultivars were provided from a collection of The Volcani Center in Israel. The presumed botanical variety of the advanced cultivars is based on a report of Malo et al. (1977).

DNA preparation. Total cellular DNA was isolated from freeze-dried leaves of individual trees by a modification of the method of Murray and Thompson (1980).

Source and labeling of probes. Variation of 18S to 25S rDNA was assayed using the rDNA clones pRZ52, pRZx, and pRZ7, from Cucurbita pepo L, covering a complete repeat unit (cf. Torres et al., 1990). Variation of 5S rDNA was assayed using the clone pHWH 5a from Petunia hybrids (Frasch et al., 1989). The probes were labelled with biotinyl 2-deoxyuri-dine 5-triphosphate (biotin-11-dUTP) by nick translation (Rigby et al., 1977).

Southern blots and detection procedure. Digestion of DNA, elec-trophoresis, vacuum-blotting, hybridization and washing of filters was performed according to standard methods (Maniatis et al., 1982). Detection of the rDNA fragments by a streptavidine-alkaline phosphatase-conjugate and a color producing substrate was according to the protocol of the manufacturer (GIBCO BRL, Gaithersburg, Maryland, USA). Fragment sizes were estimated by linear regression analysis. Standard markers were co-electrophoresed on each gel.

Results

Ten different rDNA fragment sizes could be detected in Pstl-digests of avocado DNA, ranging from 8.15 to 10.4 kb (Table 2). With the exception of 'Criollo 42', a 9.05-kb fragment was found to be present only in accessions of var. drymifolia. Similarly, fragment sizes greater than 9.6 kb were restricted to accessions of var. drymifolia, again with the exception of 'Criollo 42'. It is noteworthy that leaves of 'Tochimilco 5' lack the anise odor of var. drymifolia, but reveal a typical rDNA fragment pattern of that variety. Accessions of var. guatemalensis and var. americana always contained a 9.3 kb fragment not found in accessions of var. drymifolia. 'Criollo 42', a presumed member of var. guatemalensis lacked the 9.3-kb fragment.

Eco Rl digests of accessions of var. drymifolia yielded less uniform fragment patterns compared with var. guatemalensis and var. americana (Table 3). A 3.5-kb fragment was restricted to accessions of var. drymifolia. A 4.6-kb fragment was found in some accessions of var. drymifolia, but was not detected in accessions of both var. guatemalensis and var. americana. Instead, accessions of the latter two varieties always contained a 3.1-kb fragment not found in accessions of var. drymifolia. Exceptions are 'Aquila 4' and 'Criollo 42', which possess or lack, respectively, a 3.1-kb fragment. The occurrence of a 2.9-kb fragment in ‘Tochimilco 1' also represents a remarkable exception.

Only accessions of var. guatemalensis yielded a 0.9-kb fragment after Sstl-digestion and probing with the 18S to 25S rDNA (Table 4). Furnier et al. (1990) also reported that this fragment is unique to var. guatemalensis.

The hybridization patterns in Southern blots of Bam Hl-digests probed with 5S rDNA indicated two repeat length variants of 5S rDNA for avocado; a 560-bp repeat unit for var. guatemalensis and var. americana, and a 540-bp repeat unit for var. drymifolia (Table 4).

 

Discussion

The repeat lengths of 18S to 25S rDNA found in higher plants extend from ca. 8 kb to ca. 14 kb (Hemleben et al., 1988). It seems reasonable, therefore, to assume that the fragments generated by Pstl represent different repeat lengths of avocado 18S to 25S rDNA. Thus two types of polymorphism could be detected: (1) a repeat length polymorphism detectable after digestion with Pstl, and (2) a restriction site polymorphism detectable after digestion with Eco Rl or Sstl. It is expected from these results that digestion of avocado DNA with either Pstl and Sst I, or Eco Rl and Sstl and probing with 18S to 25S rDNA yields enough information to identify the botanical variety of an avocado cultivar. However, digestion with either Pstl or Eco Rl alone, will only allow the positive identification of var. drymifolia and will leave var. guatemalensis and var. americana un-separated as one group with similar fragment patterns (Tables 2 and 3). Likewise, digestion with Bam HI and probing with 5S rDNA reveals similar repeat lengths for var. guatemalensis and var. americana, but a different repeat length for var. drymifolia (Table 4). These preliminary results, therefore, suggest a relatively close phylogenetic relationship between var. guatemalensis and var. americana, compared to a more distant lineage of var. drymifolia. This is in contrast to Scora and Bergh (1990), who stated that the three varieties are about equally distinct from each other. It is in agreement, however, with earlier classifications, for example by Kopp (1966), who did not separate var. guatemalensis and var. americana in two distinct varieties.

The foregoing discussion fits likewise to advanced and primitive cultivars, with the exception of 'Criollo 42'. 'Criollo 42' has been identified in the field as var. guatemalensis, but it did not reveal typical fragment patterns of that variety in the rDNA assay (Tables 2 and 3), except for the presence of a 0.9-kb fragment common to members of var. guatemalensis (Table 4). It may be suggested, therefore, that 'Criollo 42' is not a member of var. guatemalensis, but may belong to the complex group of the ancestors of 'Guatemalan criollos' (cf. Schieber and Zentmyer, 1977). Moreover, 'Aquila 4', which has been identified in the field as var. drymifolia, revealed a 3.1-kb fragment typical for var. guatemalensis and var. americana (Table 3). Probably, 'Aquila 4' is not 'pure' var. drymifolia, but of hybrid origin. Tochimilco 1', an other presumed accession of var. drymifolia, showed an unusual 2.9-kb fragment in the Eco Rl digest (Table 3). This fragment has also been detected in one accession of P. schiedeana (unpublished results). In fact, some of the relatively rare fragments, as 8.15-kb, 9.2-kb, and 4.5-kb, were detectable in various accessions of P. schiedeana and other Perseas (unpublished results). It may therefore be speculated that the relatively high variability observed in rDNA fragment patterns of primitive cultivars reflects remote events of hybridizations.

In conclusion, the assay of rDNA proved to be useful in the identification of the variety of primitive and advanced cultivars of avocado. The variation of rDNA data may suggest a relatively close phylogenetic relationship between var. guatemalensis and var. americana, leaving var. drymifolia somewhat distinct.

We thank R. Torres and W. Wenzel for gifts of rDNA clones and helpful advice, and F. Bangerth for supporting this research. We gratefully acknowledge the cooperation of A. Barrientos Priego, E. de la Cruz Torres, and L. Lopez Lopez. We appreciate the financial support provided by the German-Israel Agricultural Research Agreement (GIARA).

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