Evidence of Hybridization Between Pinus attenuata and P. radiata in New Zealand
[Received by the Editor, August 21, 1956.]
Pinus radiata and P. attenuata are two species with many morphological and ecological differences. Artificial F1 hybrids between them have given rise to a vigorous F2 generation. In nature there are barriers which generally prevent crossing, but natural hybrids have been reported near Año Nuevo Point, California, the only place where the species occur together in the wild. In New Zealand they have been grown side by side for at least 80 years, and putative hybrids, including some of the F1 generation and some back-crosses to P. radiata, have been found in five widely separate places. As a basis for further study, the following hypotheses are suggested:
Introgression of P. attenuata into P. radiata has been going on for a long time near Año Nuevo Point; some of the first P. radiata grown in New Zealand may have come from seed collected there.
Spontaneous hybridization in some places in New Zealand has begun anew the process of introgression, which would tend to be accelerated by cultural practices.
Some of the variation of P. radiata in New Zealand may therefore be the result on introgression from P. attenuata.
Knobcone pine, Pinus attenuata Lemmon, is a species closely related to P. radiata, but, unlike that species, it has never been a favourite in arboriculture or silviculture. Despite this, records suggest that it was grown in some early private collections of conifers in New Zealand, especially in Canterbury, as long ago as 1870 (Adams, 1916: 221), and after 1900 it was planted in at least three exotic forests owned by the State. For some years now the tendency has been to cut down the old specimens without attempting to perpetuate the species, and consequently it has disappeared from several places where it once grew.
The only place where P. attenuata and P. radiata occupy contiguous areas in nature is near Año Nuevo Point in California, and several observers (e.g., Lindsay, 1932: 1) have reported natural hybrids there, but have provided no details about them. Artificial hybrids were produced in California in 1929, using P. attenuata as the female parent, and these were formally described as P. × attenuradiata by Stock-well and Righter (1946). These authors also reported high fertility in the F1 and vigorous growth in the F2
Many distinct differences between the species have been recorded: in their morphology (Stockwell and Righter, 1946: 159); in the chemistry of their oleoresins (Mirov and Haagen-Smit, 1949) and in their geographical distribution and ecological behaviour (Little, 1953: 261, 271; Sudworth, 1908: 58–65) The morphological differences are faithfully maintained in cultivation, except that in New Zealand the distinction of P. attenuata as having “many” cones on the trunk, and P. radiata as having “few” (Stockwell and Righter, 1946: 159), often breaks down; many trees of P. radiata here bear cones abundantly on their trunks. The pollen seasons of the two species, as recorded in California by Duffield (1953), are different, and
Text-fig. 1.—Sketch map of New Zealand, showing records of Pinus attenuata and its hybrids. with P. radiata. The round spots are for P. attenuata, and the crosses are for hybrids Asterisks indicate places where the specimens are known to have been destroyed by 1956. The F2 hybrids are of artificial origin and were imported from California.
there is a corresponding difference in New Zealand. The evidence indicates that when the two species are growing in the same habitat there is a period of about one week during which they shed pollen simultaneously.
Wherever P. attenuata has been grown in New Zealand, there have probably been opportunities for it to hybridize with P. radiata. To what extent it has done so has been the subject of a recent survey (Text-fig. 1). The following account describes putative hybrids and discusses their significance.
Putative Hybrids in New Zealand
1 Rowallan, near Darfield, Canterbury.
Single tree, found in plantation of large-coned P. radiata, planted 1917, grown from Canterbury seed. General appearance at 13 years of age reported (Hocking, 1930: 24) as rather more heavily branched than surrounding trees; mucros inflexed at base of cone, reflexed at apex. May well have been a spontaneous F1.
2. Greenvale, near Waikaka, Southland.
Two trees, found 1956 by party collecting P. radiata seed from felled shelter belt. Origin said to have been Dunedin City Corporation Reserves Department; planted about 1920. Local observer reported that they had irregular branches, thin bark. Specimens from one tree and seen by me showed: buds and scale-leaves like P. attenuata; foliage more like P. radiata; young shoots, stomata, cone shape and apophyses intermediate; umbos and mucros mostly erect, but some slightly inflexed, some slightly reflexed. (Text-fig. 3.) Probably spontaneous F1 hybrids.
3. Kainui, near Wakefield, Nelson.
One tree, dead since about 1941, found 1949 in dense stand of self-sown P. radiata which originated about 1920. One P. attenuata sapling, apparently self-sown, found less than 300 m away; previous existence of P. attenuata in the vicinity therefore almost certain. Stem of supposed hybrid quite straight, height 19.1 m (63ft); diameter at breast height 16.0 cm (6.3in); branching like P. radiata grown under similar conditions; bark thin, with shallow fissures at the butt, like P. radiata; cones 52 in number, tightly shut, adhering firmly to stem, and reflexed with their axes almost parallel to it, long and slender; basal exterior apophyses gibbous, mucros in distal part reflexed, in proximal part erect to inflexed. (Text-fig. 4.) Confidently regarded as a hybrid, perhaps an F1.
4 Karioi Forest.
Four trees, planted 1950 as arboretum specimens of P. attenuata. Originated from seed collected from single tree of P. attenuata in a row of P. radiata near Tikokino Post Office. Buds and scale-leaves like P. attenuata, other vegetative characters like P. radiata or intermediate Cones few, in April, 1956, maturing on only two of the supposed hybrids, intermediate. All four trees regarded as spontaneous F1 hybrids. (Text-fig. 5 and Plate 18, Fig. 3.) Eleven other surviving trees, derived from the same seed-lot, all typical P. attenuata.
5. Ashley Forest.
Many trees in plantations of P. radiata established 1939–40. Originated from N.Z. Forest Service seed, Lot No. C38/131, collected as P. radiata at Hanmer Forest. In 1952 some were identified as hybrids by Dr. J. W. Duffield, who was familiar with the artificial and natural hybrids in California. Varying degrees and combinations of characters suggesting the presence of P. attenuata genes—eg; forked and sinuous trunks; coarse, lax foliage; narrow, recurved cones; angular, attenuate, or grotesquely swollen apophyses; inflexed mucros. Examples of cones illustrated in Text-figs. 6–11.
These putative hybrids are judged to represent some generation later than the F1, and to consist mainly of back-crosses to P. radiata. Since this population was derived from seed collected at Hanmer Forest, it is postulated that at least one hybrid, of F1 or similar genotype, existed there. Such a hybrid could have arisen spontaneously, alternatively, it might have been of natural origin and imported in P. radiata seed.
Segregation in Progenies of Putative Hybrids
Some seed from the dead tree at Kainui (described under “3” above) was sown at Rotorua in 1954 and produced 221 seedlings. After nine months, most of these resembled P. radiata, but some showed the following characters which suggested the presence of genes from P. attenuata:
(a) leaning stems;
(b) coarse ragged appearance;
(c) apical tuft of leaves curved over to one side;
(d) purple colouring, especially of the leaf-tips in winter. One individual, presumably an extreme segregate, was strikingly like some P. attenuata seedlings grown the year before from a tree on Mt. Shasta, California. (Plate 18, Figs. 1 and 2.)
In its second year this progeny continued to show characters of both species, in various combinations. It was concluded that it consisted mostly, perhaps entirely, of back-crosses from a hybrid to P. radiata.
An even more striking segregation of specific characters was found in the progeny of one of the Ashley trees (Text-fig. 6). An analysis of this progeny is to appear elsewhere in this journal.
Possible Introgression of P. attenuata into P. radiata
On morphological grounds, P. radiata has generally been called a “highly variable species,” and there are physiological signs—e.g., in growth-rate and in the exceptionally long pollination season recorded by Duffield (1953), which also suggest an extraordinary genetic basis for its variation. Cockayne (1932: 162) suggested that it was a compound species or perhaps a linneon “with hybrids in plenty,” and others since then have suspected extensive hybridization (see Poole, 1947: 276). In a speculative discussion (Bannister, 1950: 66) it was suggested that introgression from P. attenuata might have been one of the causes of this variation. Support for this idea has been provided recently by Professor G. Ledyard Stebbins, jun. (pers. comm., 1955), who writes as follows:–
“About one mile inland from the coast at Año Nuevo Point, the P. radiata forest ends abruptly, due either to the diminished influence of the ocean fog or change in soil type, or both. Adjoining it is a chaparral of mixed sclerophyllous shrubs, with numerous trees of P. attenuata scattered through it. I have visited the area twice, once with a class. At that time, we observed carefully about 1,000 trees in the immediate vicinity of the boundary line between the two communities, and found eight which we considered to be F1 hybrids or back-cross derivatives. We judged this by comparison with cones of the artificial F1 P. attenuata × radiata which has been produced at Placerville. On the second visit I gained the impression that the entire northern forest differs from that at Monterey in the direction of P. attenuata and may therefore consist wholly of derivatives of introgression.”
Professor Stebbins has very kindly sent me three separate collections of cones, representing 26 trees near Año Nuevo Point and 31 in the Monterey area. It is apparent even from a brief inspection that the sample from Año Nuevo Point does differ from the others (see also Fielding, 1954: 12) and that the trend of its differences is in the direction of P. attenuata. Moreover, its pattern of variation is very like that noted in the Ashley stands.
The demonstration of introgression requires sensitive analysis, and an essential prerequisite for this is a satisfactory concept of the morphological norms and limits of the two species without hybrids. It may prove impossible to get this from cultivated specimens in New Zealand, but in the meantime the following hypotheses are offered as worth further investigation:
The P. radiata population at Ashley Forest shows an early stage of introgression from P. attenuata, possibly the first generation of back-crossing.
The P. radiata population at Año Nuevo Point has been modified by natural introgression from P. attenuata, and this process has been going on for many generations.
Trees derived from seed collected near Año Nuevo Point have played an important role in the genesis of present-day stands of P. radiata in New Zealand.
Text-fig. 2.—Cone of Pinus attenuata from a tree in Blenheim.
Text-figs 3–10—Cones from eight different trees showing signs of hybridism Fig. 3—From Greenvale, probably an F1 Fig. 4—From a dead tree at Kamui Fig. 5—The first cone to ripen on the F1 hybrids at Karioi Forest Figs. 6–10—All from Ashley Forest The last one came from a tree which still stood after the thinning.
In connection with this last suggestion, Professor Stebbins continues:–
“When we were at Año Nuevo Point we talked with a farmer who lives at the edge of the forest. He said that he and the previous owner of his farm have been sending seed to all parts of the world for many years. It seems to me that some of the attenuata-like trees in the New Zealand forests may have come from seeds of this origin.”
Although it is 40 years or more since a significant amount of P. radiata seed was imported into New Zealand, it does seem probable that some of the earlier imports were from the area around Año Nuevo Point.
In several recently visited stands of P. radiata, where there was circumstantially no reason to suspect that hybridization had occurred, there were odd trees which showed more or less resemblance to P. attenuata. The most reliable clues to the ancestry of these trees were probably provided by their cones. In the examples illustrated, the characters noted particularly were:
Text-Fig. 12—Cone strongly reflexed on its peduncle; axis slightly recurved near the apex; umbos erect or distinctly inflexed.
Text-Fig. 13.—Cone strongly reflexed; umbos erect to inflexed.
Text-Fig. 14—Cone extremely reflexed; axis more recurved than usual.
Text-Fig. 15.—Axis recurved near apex; apophyses on outer side extremely long, attenuate.
Text-Fig. 16—Cone extremely reflexed; axis lying almost parallel to supporting branch.
Trees such as these cones represent are not necessarily regarded as possessing P. attenuata genes, but their presence is consistent with the hypothesis that introgression occurred in bygone generations.
Introgression in Relation to the Cultivation of the Species
In nature, the supposed flow of genes from P. attenuata into P. radiata may perhaps be likened to diffusion through a restricting membrane. Several circumstances hinder this flow: the two species have different pollination seasons; they occupy mainly separate territories; habitats favourable to the survival of F1 or similarly intermediate hybrids are probably few and of small extent; and seeds seldom spread very far from the mother tree.
In cultivation, on the other hand, most of these restrictions would be removed, and this would probably result in what may be termed accelerated introgression. In New Zealand, in most places where P. attenuata grew, P. radiata would be close at hand; hybrid seed which happened to be included in a P. radiata seed-lot would have excellent chances of growing to reproductive maturity; and the mixing of divers progenies which occurs in a normal seed-lot would ensure that the progeny of a hybrid would be widely dispersed throughout the resulting population.
For example, if seed were collected from 50 trees in a shelter belt, 49 of them P. radiata and the other an F1, it is estimated, very roughly, that the resulting population when planted would cover 500 acres and would contain 10,000–20,000 hybrids distributed at random. Out of an average of 20–40 hybrids per acre, some would be expected to survive and make their contribution to the next generation. If only one hybrid per acre survived, produced 1,000 viable seeds, and was the father of 1,000 others on the adjacent P. radiata, then there would be a million seeds each containing P. attenuata genes. It is suggested that such a dispersal of genes, from a single P. attenuata gamete, could easily take place in 50 years in cultivation, and that many of these genes would persist indefinitely in subsequent generations.
It can be argued that the P. radiata trees would tend to suppress and kill the hybrids and so eliminate them from the breeding population; it can also be argued that thinning would produce the same result, because P. attenuata genes are likely to have an obviously adverse effect on the shape of the stem. There is probably truth in both these contentions, but it should be noted that the artificial F2 reported by Stockwell and Righter (1946: 158) included trees which were as vigorous as P. radiata; and furthermore, some hybrid derivatives may possess all the visible characteristics of a desirable tree, and so would be favoured, not removed, in the
Fig. 1 (left)—Extreme specimen among 221 seedlings, from a supposed hybrid at Kamui
Fig. 2 (right)—Herbarium specimen of a P. attenuata seedling, typical of those from a tree on Mt Shasta, California (Background in 1 cm squares)
Fig. 3—Group of trees in the arboretum at Karioi Forest The small pale one in the centre is P. attenuata, the darker ones to right and left of it are supposed F1 hybrids, P × attenuradiata
Text-figs. 11–16.—Cones which suggest, in varying degrees, introgression of P. attenuata into P. radiata. They were collected at the following places:—Fig. 11—Ashley Forest, Compartment 3 Fig. 12—Waiotapu Forest, Compartment 7. Figs. 13–16—Kaingaroa Forest, Compartments 1058, 1125, 1038 and 1059 respectively.
thinning operation. At Ashley Forest, where a first thinning of P. radiata was carried out in the 15th year, some of the remaining trees had characters strongly suggesting hybridity (e.g., Text-fig. 10).
There is another possible difference, where P. radiata and P. attenuata are concerned, between introgression in nature and introgression in cultivation. In nature, there has been only one recent source of P. attenuata genes, and that is the local population near Año Nuevo Point. In cultivation, some of the P. attenuata specimens involved in hybridization may have been derived from a different geographical source, such as the Sacramento Valley or the Sierra Nevada; such trees might well be genetically different from those of coastal origin. If so, introgression in cultivation may in some places have caused unprecedented enrichment of the variability of P. radiata.
If the hypothesis of ancient and continuing introgression is correct, it has an important bearing on the culture of P. radiata in New Zealand. There may be genes derived from P. attenuata, and now included in the gene-pool of P. radiata, which have no effect whatsoever on visible form, yet play a vital part in metabolism. Such genes may be helping P. radiata to exist, and even to thrive, in places to which it might otherwise be less well adapted. As Anderson (1949: 109) said:
“Introgression is of the greater biological significance, the less is the impact apparent to casual inspection.”
Note.—Since this paper was written, a specimen of P. attenuata has been found near Utiku, in the Rangitikei district, and records have been noted, in unpublished reports, of the species having occurred at Lake Coleridge, Mount Peel, Winchester and Otaio, all in Canterbury. Some trees in Maramarua Forest, in the Auckland district, were recently reported to me as P. attenuata. On examining these, I concluded that they were all atypical of that species; at least two of them were more like the F1 hybrid, P. × attenuradiata. Progeny tests of some of these trees are now in progress, to determine whether they are hybrids or nto.
My thanks go to Professor G. Ledyard Stebbins, jun, for his interest and help, and for permission to quote from his letter; to Mr. C. M. Smith, for an illuminating dissertation on the word “thinning”; to Dr. E. J. Godley and Mr. G. C. Weston for helpful comments on the manuscript; and to all those officers of the New Zealand Forest Service who helped with the field work.
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