157 TANE (1967) 13 : 157 - 165

INTROGRESSIVE HYBRIDIZATION AS ILLUSTRATED BY AREOLATA (CHEESM.) AND COPROSMA RHAMNOIDES (A.CUNN.) (.)

By Jennifer J. Bedford *

INTRODUCTION

The analysis of the problem of the species in and of the related dynamics of natural populations may be carried out by using a com• bination of the techniques of taxonomy and genetics to study species diff• erences, variations in populations and of the evolutionary forces at work in speciation. This is the method reported in this article.

Coprosma areolata and C. rhamnoides are found in forests through• out the north and south islands of New Zealand. C. rhamnoides has a more extensive distribution as it is found in scrub areas as well. C. areolata is a tall (up to 5 m.) fastigiate shrub in which the veins of the leaves form a conspicuous network. C. rhamnoides — a polymorphic species, (Allen, 1961) is smaller (up to 2 m.) and divaricating with no such obvious venation as in C. areolata.A fuller more detailed description of both species is given by Allen (1961). The two species differ predominately in growth form, details of the leaf tip, leaf venation, and leaf size. Both species however hybridize with other members of the genus. (Allan records C. rhamnoides crossing with C. lucida, C. rotundifolia, and C. repens but does not mention hybrids between C. rhamnoides and C. areolata.) Therefore often one specimen may have more than two species detectable in its phenotype.

The two species only may be found together but usually there may be from three to six species of Coprosma growing in the same locality or the same direction of the prevailing winds so that multiple hybridization may result.

In localities in which only these two species are found marked introgression is seen. Anderson (1953) defined i ntrogressive hybridization as "a gradual infiltration of germ plasm of one species into that of another as a consequence of hybridization and repeated backcrossing."

There are a number of ways in which introgressive hybridization may be represented in natural populations. (Pig. 1)

1. The two populations may remain as distinct entities with no indication of hybridization between them. 2. One population (A) may introgress into another population (B) i.e. the genes of one population infiltrate into those of another and there is very 'Department of Genetics, University of Auckland

Present address: Dept. of Zoology, University of Auckland 158

FIG.1. POSSIBLE WAYS IN WHICH INTROGRESSION Mo.Y BE REPRESENTED. 159

little reciprocal backcrosslng — the genes going in one direction only. Initially only a small number of true hybrids are formed. This infiltration may be from A to B or from B to A. 3. Introgression may be a two-way process resulting in a small number of hybrids again but gradually the number of hybrids increase until a hybrid swarm is reached.

Which of these possibilities may have occurred in the hybridi• zation of C. areolata and C. rhamnoides will be discussed later.

LOCALITY

There are a few areas in which Coprosma areolata and C. rham• noides are the only species of Coprosma present. Where the two meet they hybridize freely. It was therefore necessary to find populations of the two species — relatively or completely free from any other interfering .

One such area chosen was Smith's Bush which as far as could be ascertained consisted only of C. areolata and C. rhamnoides. However the population here was considerably disturbed by the construction of the northern motorway through one part of the reserve. Previously cattle had been allowed through and much of the young undergrowth had been des• troyed by trampling and grazing. Both these factors disturbed the equi• librium of the ecological community.

The other locality was in Glenfield where although other Coprosma species such as C. robusta and C. arborea were present — most of the hybrids were between C. areolata and C. rhamnoides. Compared with Smith's Bush which is possibly drier, level, and fairly open, and with a high canopy of Podocarpus dacrydioides, Melicope turnata, Vitex lucens, and Lepto- spermum scoparium, this property slopes sharply down to a small stream with predominately Dysoxylum spectabile, Rhopalostylis sapida, Cyathea dealbata, a few Beilschmeidia tarairi, Podocarpus totara, and Phyllo- cladus trichomanoides.

CHARACTERISTICS USED IN THE ANALYSIS

About sixty species were taken from each locality. Personal bias was eliminated as much as possible by selecting every tenth along the track (which by its very nature is a disturbed habitat). The three measur• able and quantitatively variable factors between the two species are leaf length, leaf width, and internode length. Generally in C. rhamnoides the leaf length and internode length are less than in C. areolata. The leaf width in both species — although smaller in C. rhamnoides — shows less difference. Fifty leaves from each plant selected were taken, the width and length measured, and the mean estimated. The internode length measured COPROSMA AREOLATA X C, RHAMNOIDES.

i 1 . _ ^ .

-r-i ; : ; 1 * * * * * s 1 =r LEAF LENGTH (in mm). 161 was the second internode on the branchlets off the main stem. Others proved to be too variable within a single plant.

Branch angle, phyllotaxy, leaf thickness, and berry colour and size did not prove reliable in determining differences between the two species.

Other characteristics which were used were classified on a semi• quantitative basis. These were leaf tip, (attenuate, acute, and round) and venation (conspicuous, faint, and obscure.)

The results were initially plotted as pictorial scatter diagrams and as frequency curves. Hybrid index analysis was later used and consists of assigning arbitrary values to the different characters of the plant used in the analysis. Once the value has been calculated frequency curves and histograms may be constructed. (Values were so chosen so that C. rhamnoides had a total value of 0, and C. areolata 100.)

It should not be inferred that two characters are equally import• ant because they have the same index value. Nor should a genetic similarity be assumed if two have the same total value. This does not mean they have similar characteristics but rather that both plants result from a similar degree of introgressive hybridization and backcrossing.

RESULTS AND DISCUSSION

I. The Environmental Influence on the Plants: There is firstly the influence of exposure and shelter and the aspect of the locality. With greater exposure the plants (especially C. rhamnoides) are more compact and divaricating. This is also reflected in the thickness of the leaves — those in more exposed conditions having thicker leaves than those in shelter. Age is also a factor. As C. areolata becomes older it becomes more fastigiate, and C. rhamnoides — more di• varicating.

Secondly there is the interference of the environment by man. Glenfield is an example of regenerating bush and hybridization is enhanced by this interference. However on a more drastic scale as is seen in Smith's Bush with the construction of the motorway through one portion — the damage may be irreversible — disturbing the entire equilibrium of the forest. Excess light and wind may be let in — disturbing previously protected seedlings, etc. In Smith's Bush, cattle had been permitted through and thus had destroyed much of the undergrowth. (In regenerating bush, the equi• librium is initially upset but in time is gradually regained offering more opportunities for hybridization.) COPROSMA AREOLATA X C.RHAMNOIDES. Scatter diagram plotting leaf length against i 1 internode length. This shows introgressive attenuate acuta round conspicuous feint obscura. hybridization of the two characters concerned. J «s J v A similar diagram is obtained when length is leaf tip venation replaced by width.

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\ r- m S L O -4 X L Co 5' U L L u l_ i—

a

1 » s—«- ; * *——* * fc -tr—C—JS S ft C J7- INTERNODE LENGTH (in mm). 163

Thirdly there is the direction of the prevailing winds. This is important, for the population of one species situated in the path of the prevailing winds may either have pollen brought to it or it may contribute pollen to another population (Coprosmas are wind pollinated).

II. The Genetic Factors Involved in Hybridization: (Fig. 2-4) In the pic• torial scatter diagrams leaf length and leaf width are strongly correlated (isometric) although the mean curve is slightly sigmoid. This is more noticeable in the graphs of the two separate localities. In Smith's Bush growth appears to be mainly isometric with a slight tendency towards being allometric. At Glenfield this tendency towards allometry is more marked. In the overall graph growth is allometric and the leaf width increases as does the leaf length but not in exact correspondence. The degree of increase in the leaf width is less than that of the leaf length. Leaf length and thus leaf width show marked introgression when plotted against internode length (Fig. 2).

Frequency curves using hybrid index values show two marked peaks — one approximately at "true" C. rhamnoides and one at about "true" C. areolata. There is an overall similarity in the graphs — whether the characters are taken five at a time (leaf length, leaf width, internode length, leaf tip and leaf venation) or three at a time (leaf length, leaf width and internode length). There is a dip in the curve between these two peaks and this represents the first generation hybrids. In the two populations plotted seperately — that at Glenfield shows a higher peak for C. rhamnoides than does Smith's Bush. The reverse is so for the latter. This generalization stands regardless of the characters used in the analysis.

Smith's Bush may have pollen of C. rhamnoides blown to it from populations east of it. Thus pollen of C. rhamnoides would be introgressing into that of C.areolata at Smith's Bush (while the reverse appears to be so of Glenfield) . Glenfield has not yet reached a stable state of equilibrium or a hybrid swarm . Smith's Bush on the other hand, may have passed this stage and be a hybrid swarm backcrossing to an original parent species . This is quite possible as about ten to fifteen years ago this population was more or less a complete hybrid swarm (J.A. Rattenbury — pers. comm.). Now it appears to be mainly C. areolata.

In frequency curves of characters analysed seperately and taken three at a time for leaf length and leaf width, and four at a time for inter• node length — three appeared to be seven or eight peaks which indicates there are three (possibly two) genes involved in the phenotype of a single characteristic. Venation and leaf tip analysis show a similar pattern to those just mentioned — two major peaks around C. areolata and C. rhamnoides and a smaller middle portion of hybrids. 164

a w Hf&e-iD irgoex vaujss FIG 4- Coprosma areolata x C. rhamnoides. Frequency curves of characters taken five at a time (using hybrid index values). (Solid line represents the total population, the dotted line — Glenfield, and the broken line — Smith's Bush.)

From the results it appears that in the case of Glenfield, genes from C. areolata may have introgressed into those of C. rhamnoides. The reverse is so with Smith's Bush — C. rhamnoides infiltrating into C. areolata — however, the results obtained from here may not be quite as definite as the motorway has disturbed the habitat.

In this analysis C. rhamnoides has been considered as an un• varying species and not as a plastic one. This plasticity definitely com• plicates the analysis of the hybrid population. 165

CONCLUSIONS

Both species show marked introgression — one to the other — which has been faciliated by man's interference. The population at Glenfield seems to have arisen by introgression of C. areolata into an original popu• lation of C. rhamnoides with very little backcrossing .That at Smith's Bush may originally have been C. rhamnoides, later reached the stage of a hybrid swarm between C. rhamnoides and C. areolata, and finally, by the action of natural selection, showing more introgression of C. areolata into this popu• lation so that now this area is predominately C. areolata.

ACKNOWLEDGEMENTS

I wish to thank Dr J.A. Rattenbury for his help and supervision of the original project on which this article was based.

REFERENCES

Allan. H.H. 1961 Flora of New Zealand, Vol. 1. Govt. Printer, N.Z. p. 559 ff.

Anderson, E. 1949 Introgressive Hybridization. John Wiley & Sons, Inc.

1953 Introgressive Hybridization. Biol. Rev. 28: p. 280.

Forde, M.B. 1962 Effect of Introgression on the Serpentine Endemism of Quercus durata. Evol. 16: p. 338.