INTRODUCTION

Two species of Macadamia are cultivated for their palatable seeds, which marketed variously as "Macadamia nut" "Australian nut", ''Queensland ", "bush nut", and ''Bauple'' (sometimes spelled "Bopple") nut". The species are M. tetraphylla, L. A. S. Johnson, and M. integrifolia Maiden and Betche. The species are indigenous in the subtropical region of Australia east of the Great Dividing Range. Both were "domesticated" about 90 years ago. M. integrifolia is now grown extensively in Hawaii as a commercial source of nuts. M. tetraphylla and M. integrifolia are cultivated in California, Rhodesia, Kenya, South Africa, and elsewhere, as well as in their native Australia.

The natural range of M. tetraphylla extends northward from the Richmond River in northeastern New South Wales to the Coomera and Nerang Rivers southeastern Queensland, a distance of about 75 miles. The southern limit is at about 29 S. Lat. and the northern limit is at about 28 S. Lat.

M. integrifolia is strictly a Queensland species with a natural range extending from the Numinbah Valley in the south to Rockhamptom in the north, a distance of about 300 miles. The latitudinal limits are 28 S. Lat. 23 S. Lat.

The ranges of the two species overlap in a region about 20 miles wide in southern Queensland.

Johnson (1954), Beaumont (1956), and Smith (1956) reported that occasional naturally occurring trees having every appearance of being hybrids are found in this region and slightly beyond to the north. They assumed such trees to be hybrids between the species. Johnson (1954) stated "The specimens exhibit considerable variation, which suggests the existence of a hybrid swarm, involving more than one generation of hybridize. The hybrids show intermediate conditions in petiole length, leaf shape and dimensions, number of marginal teeth and of lateral nerves, roughness of the seeds, and in general aspect, flowers have not been seen. The number of leaves in the whorl is usually three, occasionally four. Variation on one plant may be considerable." Beaumont (1956) noted "The two species overlap in the Guanaba and Tambourine Creek Valleys, and in ‘The Pocket’ in South Queensland, a distance of hardly more than 5 miles on the eastern slopes of the mountains. It was in this area that intermediate types between the typical species forms, as well as typical representatives of the species, were found growing together under natural conditions. The intermediate types, which doubtless arose by hybridization, would be extremly difficult or impossible to classify as one or the other species." Smith (1956) wrote, "On Wongawallan, Tamborine, Guanaba, and Clagiraba Creeks all tributaries of the Coomera River, M. integrifolia and M. tetraphylla occur together naturally. Characters exhibited by a number of trees examined in this area suggest that some interchange of genes has taken place between two species, while a few trees may have been F1 hybrids. Among cultivated trees in orchards, individuals are found displaying intermediate characters. These may have arisen as a result of the two species having been cultivated in one orchard."

One of us (W. B. S.) was shown a number of trees with intermediate appearance in the vicinities of Gilston and Beachmont during a visit in Australia in 1960. He agrees with the persons mentioned above that these very likely had a hybrid origin. Beaumont sent Scionwood of several putative hybrids to Hawaii and California in 1953. This was propagated successfully by topworking on well-established, vigorous trees in a seedling orchard. Observations on this material in the course of development over the past several years tend to support the supposition of hybrid origin.

Each of the species has certain desirable horticultural characteristics lacking in the other. We know of a number of attempts to hybridize them, including several by the senior author, in the hope of combining the desirable characteristics in the F1 progeny or, failing here, of obtaining individuals in the F2 which have inherited these characteristics through recombination. These attempts at hybridization came to naught. We know of none, which succeeded, nor have we seen anything in the literature, which would indicate that someone has been successful in achieving hybridization.

In 1956, the senior author chanced upon a young orchard of 96 seedling Macadamia trees on the Rancho Santa Fe, California. These were typical neither of M. tetraphylla nor of M. integrifolia but rather were intermediate in appearance between the two species, suggesting the possibility that they might be hybrids. The supplier of the orchard trees was the nurseryman who had propagated them. He took the senior author to his source of seed. This turned out to be a tree of M. tetraphylla on the F. W. Robertsen property in Santa Ana, California. Less than 25 feet away was a tree of M. integrifolia. Both trees were large with considerable interlacing of branches. They appeared to be about 50 years old, and quite probably were planted at the same time. About 1968 they were cut down to make room for a commercial building.

Since the original discovery, the senior author has located an additional 50-60 seedling trees in scattered localities, all from the same source and having the same characteristics.

The nuts of M. tetraphylla tree were large and attractive in appearance, with kernels of good quality. Consequently, it attracted the attention of nurserymen, who raised seedlings and disseminated them widely. Every seedling appeared to be hybrid, suggesting that natural cross-pollination had been taking place. The nuts of the M. integrifolia tree were not impressive, consequently virtually no attention was paid to it. No nuts from it are known to have been planted, so we have no way of knowing if reciprocal hybridization might have occurred.

These paper reports on comparisons of taxonomic characters and cytological observations, which tend to confirm the assumption that the seedling progeny at Rancho Santa Fe resulted from hybridization between the two species. The similarity of the individuals of this progeny to trees occurring naturally in a is strong circumstantial evidence that the latter were of inter-specific hybrid origin.

TAXONOMIC CHARACTERS

M. tetraphylla. The tree of this species was the female parent of the putative hybrid progeny. It was typical of the species as a whole, with sessile, lanceolate, serrate leaves borne in nodal whorls of 4, pericarp dehiscent on the tree, and pebble-shelled seeds, shown in Fig. I, A.

M. integrifolia. This tree most probably was the pollen parent of the hybrid progeny in view of its close proximity to the M. tetraphylla tree. No other tree of the species is known within the radius of one mile of the site. It, too, was typical of its species, with oblong, entire or slightly dentate, petiolate leaves which are borne in nodal whorls of 3, pericarp which usually do not dehisce until the fruit has fallen to the ground, and smooth-shelled seeds (Fig. I, C). A raceme in anthesis is shown as Fig. 2.

M. tetraphylla x integrifolia. The F1 hybrid population is highly uniform in general aspect. There is, to be sure, a certain amount of variation among the seedlings in phyllotaxy, petiolation, numbers of marginal serrations on the leaves, intensity of the pink coloration of the perianth, size of fruit, and other characters. This variation is minor, however, and actually, is no greater than that to be found in natural populations of each of the species. A typical leaf, as well as the dehiscent fruits and the slightly pebbled seeds are shown in Fig. I, B. The node shown in this figure have 3 leaves, but 4-leaved nodes are equally common both on a given individual and among the trees making up the population. The raceme with the flowers in anthesis is shown in Figure 2. The trees appear to be more vigorous than non-hybrid seedling trees of the species of comparable age. They produce crops comparable in volume to the crops produced by the parent trees, indicating full fertility despite their interspecific hybrid origin.

Distinguishing characteristics of the two species and the presumed hybrid are listed in Table I.

One should bear in mind, that the observations summarized above, apply only to this particular Fl progeny and its parents. Differences occur in some characters among individuals in a species itself, e.g. some forms of M. tetraphylla are known which lack reddish color in the new growth, and which have white or cream colored flowers; similarly, some forms of M. integrifolia have new growth tinged with reddish or bronze coloration. Hybridization between different forms of the two species probably would yield results not like those reported above in every respect.

CYTOLOGICAL OBSERVATIONS

Chromosome numbers. M. tetraphylla, M. integrifolia, and the interspecific F1 hybrid have identical chromosome numbers of n = 14 and 2n= 28. The gamete numbers were determined from meiotic metaphase I and metaphase II figures in pollen mother cells, the mitotic numbers both from rooted cuttings and from germinating seeds. Use of rooted cuttings seemed indicated, especially in the case of M. tetraphylla, in view of the high percentage of seed result from outcrossing. The numbers reported here correspond with those reported previously for M. integrifolia by Urata (1954) under the name M. ternifolia var. integrifolia and by Ramsay, (nee Lancaster) (1963: reported in Darlington and Wylie, 1955, as an unpublished determination by Lancaster). The senior writer determined the numbers for M. tetraphylla and a closely allied species, M. ternifolia F. Muell, in the course of another study (1965).

1. Acetocarmine: Normal pollen grains treated with I .0 percent carmine in 45% acetic acid turn bright red, abnormal grains fail to absorb the stain. The grains of M. tetraphylla, M. integrifolia, and the hybrid are similar in size and structure. As noted above, all three taxons have some nonfunctional grains, the percentage being less than 1 .0 in every case.

2. TTC: A second test involving a color reaction was the use of 10 grams of 2,3. 5-triphenyltetrazolium chloride (TTC) in 1 liter of water as a stain. Resting pollen and abortive pollen give no reaction. Good pollen grains, which have had the opportunity to begin germination, however, acquire a crimson color due to activation of the enzymes present in living cells, which react with the tetrazolium salt to form an insoluble red dye. The results were comparable with those reported for acetocarmine in which functionality is of a high order.

3. Pollen germination: Macadamia pollen germinates well in a 15 percent aqueous solution of sucrose in a hanging drop, or by the several methods, used by Urata (1954), beginning 2-3 hours after sowing. Pollen germination confirmed the staining tests regarding functionality, with percentages for the two ties and the hybrid approximating 90 by the twenty-fourth hour.

OTHER OBSERVATIONS

Viability of F1 Seed: Seeds of the F1, which were assumed to result either from self-pollination of sibmating, germinated with results no different from those of seeds from the parent species. With random lots of seeds, germination ranged from 80 to 96 percent. With lots selected for good size and freshness, 100 percent germination was usually achieved.

Self-pollination: Macadamia has a unique method of pollen dispersal. Urata (1954) describes this in detail. Briefly, in the maturing flower bud 1-2 days prior to anthesis, the locals of the four anthers which are appressed to the short, developing style near its tip, but below the stigmatic surface, dehisce and deposit their complements of pollen on the style. In the days immediately following, the style begins to elongate, exerting pressure against the perianth and splits it open, freeing itself. Finally, when the flower is in anthesis, the style straightens out, bearing its load of pollen aloft (Fig. 3). There the pollen remains until it is harvested and carried off by honeybees or until it is knocked off by wind or some other mechanical means.

As noted by Urata (1954), if undisturbed, the pollen germinates on the style in about 20 hours, and can effect self-pollination. Of the 100-300 flowers on a raceme, only -20 mature into fruit. All trees studied by Urata were self-compatible, although he noted differences between trees in the ability to set fruit from self-pollination.

Following the procedure described by Urata (1954), the senior writer bagged 25 racemes each bearing 250-300 flowers just prior to anthesis of the first flowers on the parent M. tetraphylla tree. Periodical inspections disclosed a high degree of pollen germination on the styles of the flowers soon after the onset of anthesis in a raceme until its termination in 9-10 days. Significantly, not a single fruit set among the 6000-7500 flowers involved. Unbaked flowers set -8 fruits to the raceme, presumably because of pollen received from the adjacent M. integrifolia tree.

In the light of this experience, one can only conclude that the M. tetraphylla tree, insofar as is known now, is completely self-compatible.

Graft Compatibility: Studies by Storey and Frolich (1964) showed that the two species and the presumed hybrid are mutually graft compatible whether used as rootstock or scion. Furthermore, all three taxons can be easily grafted to a third species, M. ternifolia F. Muell.

NSW 44, a superior Australian selection introduced into California by way of Hawaii in 1958 under the label HAES 695, has tree characters which would lead one to identify it as a hybrid. It is more or less typical of similar trees occurring naturally in the region of overlapping of the natural ranges of the species in question. If one were unaware of its origin, he could easily infer it to be a seedling with the same or similar parentage to the other seedlings of the type, which are scattered around the State. One would not be too far amiss, therefore, in supposing that confirmation of the hybrid origin of the California seedlings can be extrapolated to the heretofore-putative hybrids in Australia.

CONCLUSIONS

The observations and investigations reported above lead one to the inevitable conclusion that seedling trees in California which appear to be hybrids between M. tetraphylla and M. integrifolia almost certainly do represent the F1 hybrid of these species. The points of evidence follow:

1. The known source of seed. This was a tree of M. tetraphylla growing in close proximity to a tree of M. integrifolia, which undoubtedly is the only reasonable source of pollen.

2. Enforced hybridity, due to the inherent self-incompatibility of the M. Tetraphylla tree, which precludes any probability of self-fertilization of the flowers.

3. Intermediacy of aspect between the two species.

4. Dominance of some characters of one parent or the other, and lack of dominance in other characters. The intermediacy of point 3 above is the reflection of this intermixing of parental dominance in some characters coupled with lack of dominance in others.

The above points of evidence of interspecific hybridize rest, of course, upon the assumption that the species in question are valid species. There seems to be no reason to doubt that they are in view of the great many differences in taxonomic characters, most, of which would be good diagnostic characters for delineating species by most botanists. Recognition of the two as distinct species goes back to Maiden and Betche (1897) who described M. integrifolia, and rests with Smith (1956) who dispelled the confusion in identity and the mix-up in nomenclature which had developed in the years intervening, we are in full accord with Smith’s precepts.

From cross-compatibility, viability of the seed, similarity of chromosome numbers, full fertility of the hybrid by virtue of normal gametogenesis indicating perfect or near-perfect chromosome homology, full viability of the F1 seed, and graft compatibility, the closeness in relationship of the two species is quite apparent. The obvious deduction is that the two species derive from an immediate common ancestor, or that one derived from the other. Species differentiation came about through accumulations of gene changes which, in no way, affected the homology of the chromosomes and their ability to pair and disjoin normally in meiosis.

Observations on the species in their indigenous regions and the findings reported herein open the way for some thought provoking questions. Why, for example, if hybridization between the two species is possible, are so few hybrids found in nature and these only in the region of overlapping of the indigenous ranges? Can it be that the nature of carrying the pollen on the style and of pollen germination in this position, block effective pollination from outside sources? This might be an explanation, for as Beaumont (1958) noted ''…wild seedlings in an isolated valley resembled each other closely, while those in another area resembled each other but differed from the first group.

Another matter for conjecture is what restricts the two species to the natural ecological niches they occupy? Each species, when transplanted anywhere in the natural range of the other, grows and thrives just as though it was at home. And yet, under natural conditions, the southern extremity the natural range of M. integrifolia terminates abruptly in southern Queensland at the Macpherson Mountain Range. The range of M. tetraphylla tapers off in the north in the southernmost part of Queensland, and in the south in the general region around the Richmond River near Lismore, N.S.W. Why the natural distributions of the two species are so definitely restricted, and why the region in which they overlap is so confined when, apparently, they are adaptable to similar conditions of climate and soil poses an interesting study in plant ecology. Superimposed on this is the problem of why, in the face of interspecific hybridize and the complete viability of hybrid seed, to which is added the longevity of Macadamia the hybrids, rather than being well distributed within the combined ranges of the species or even transcending it are restricted to the small region of overlapping which marks termination of the natural ranges of the species in question.

2. 1958. The Macadamia in Australia and Hawaii. Calif. Macadamia Soc. Yrbk. 4:25-29.