Somatic Chromosome Number of the New Zealand Broom Genera and a Discussion of Their Relationship.
Botany Department, University of Otago.*
[Read before the Otago Branch, April 8, 1952; received by the Editor, April 16, 1952.]
The somatic chromosome number of the New Zealand broom genera, Carmichaelia, Chordospartium, Corallospartium and Notospartium is 32.
It is shown that 2n = 32 is in better agreement with the somatic chromosome numbers of the Galegeae than the previous record of 30 in Carmichaelia.
The systematic problems of the four New Zealand broom genera are briefly discussed, and it is suggested that Corallospartium and Chordospartium be included as two further subgenera of Carmichaelia, but that Notospartium should be retained as a distinct genus.
An investigation of the cladode anatomy of the New Zealand brooms (12) led the author to suggest:
That the eight subgenera of the genus Carmiehaelia are very closely related to the other three genera, Corallospartium, Notospartium and Chordospartium, and.
That the genus Chordospartium might have arisen as a hybrid between the genera Corallospartium and Notospartium, since this genus shows a combination of characters found in the two supposed parental types.
It has now been found that species of all four genera have the same chromosome number of 2n = 32, and, while this does not assist the taxonomist directly, since wider groups of plants than genera may have the same chromosome number, it does exclude the necessity of keeping all these genera distinct if a closer relationship is otherwise indicated. The morphological features of the four genera have therefore been compared in order to determine whether the distinction should be preserved or whether a regrouping would be advisable.
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Radicles from germinating seeds were used when they were 3/4–1 inch in length in order that they should be free from starch.
Fixation in acetic alcohol followed by aceto-carmine or Feulgen squashes (6) did not give satisfactory preparations, nor did the modified aceto-carmine methods used by Thomas (14) and Brown (2) for small chromosomes. The following method was therefore used:—after pretreatment with a saturated aqueous solu-
[Footnote] * Part of this work was carried out in the Botany School, Cambridge University, England.
tion of paradichlorobenzene for two hours (9) the root rips were fixed in Navashin's fluid, macerated with N.HC1 at 60° C. for 30 minutes, squashed and stained with crystal violet using the schedule of Darlington and La Cour (6).
b. Chromosome Number
|Genus and Subgenus.||Species.||Somatic Chromosome Number.||Number of Metaphase Plates examined.|
|Notospartium||No. carmichaeliae Hook.f.||32||3|
|Chordospartium||Ch. stevensoni Cheeseman||32||4|
|Corallospartium||Co. crassicaule J. B. Armstrong||32||4|
|i. Monroella||Ca. monroi Hook.||32||4|
|ii. Petriea||Ca. petriei Kirk||32||3|
|iii. Carmichaeliella||Ca. williamsii Kirk*||32||3|
|iv. Kirkiella||Ca. kirkii Hook.†||32||3|
It can be seen from Table I that all the seven species studied had the same chromosome number. Seed of the other four Carmichaelia subgenera (Thomsonella, Enysiella, Suterella and Huttonella) was not available, but it is probable that their chromosome number is also 32, since both interspecific aneuploidy and interspecific polyploidy in the Leguminosae are rare (10).
On the other hand Kreuter (9) found 15 bivalents in the pollen mother cells of Carmichaelia australis var. stricta, but it is uncertain to which species in Simpson's monograph (13) this belongs, since Carmichaelia australis has been subdivided to form several species of Carmichaeliella.
c. Chromosome Morphology
All late prophase and early metaphase chromosomes were observed to be rodlike structures approximately 1.25–1.5 microns long (Fig. 1 and Plate 1), and morphologically indistinguishable except for two which had a central achromatic region (perhaps a nucleolar organiser constriction). Only four metaphase plates revealed both of these chromosomes, but it may be that the small unstainable area can only be detected when the chromosomes are oriented at right angles to the poles. They were observed in Ca. monroi, Ca. kirkii, Ca. petriei and Notospartium carmichaeliae. The primary constrictions or centromeres of the chromosomes were not discernible.
d. Chromosome Numbers in the Galegeae
Chromosome numbers in the Galegeae are shown in Table II. It can be seen that eight is the most frequent basic number. This would suggest that in the New Zealand brooms, 2n = 32 is in better agreement with chromosome numbers of the Galegeae than Kreuter's figure of 2n = 30. The nucleolus drawn in one of his figures might have obscured a sixteenth pair of chromosomes. Moreover, Carmichaelia is the only member of the Galegeae listed by Darlington and Janaki-Ammal (5) which has a basic number of x = 15.
[Footnote] * Shows advanced anatomical characters (See 12).
[Footnote] † Shows primitive anatomical features (See 12).
|Genus.||Haploid No. n—||Totals of Species.|
|Haploid No. n—-|
|Genus.||6||7||8||10||11||12||16||20||22||24||32||36||48||Totals of Species.|
|Totals A and B||2||4||39||13||5||3||9||1||1||7||5||1||1||89|
Total number of genera is 19; number of genera with x = 8 (n = 8, 16, 24, 32 or 48) is 14—i.e., 73.7%.
Within the 14 genera with x = 8, total number of species is 73; number of species with x = 8 is 61—i.e., 83.6%.
Within the whole 19 genera, total number of species is 89; number of species with x = 8 is 61—i.e., 68.5%.
e. Chromosome Numbers in the New Zealand Leguminosae
In a tentative chart of hypothetical phylogenetic relationships of the Leguminosae, Senn (10) suggested that the Papilionatae diverged from a primitive ancestral stock with a basic chromosome number of x = 8. Of the eight genera of Leguminosae indigenous to New Zealand the six belonging to the Galegeae retain this basic number and are either endemic (the broom genera*) or confined to the Australasian region (Clianthus and Swainsona). The remaining two are Sophora and Canavalia. Atchison (1) found that the diploid number is 18 in two of the species of Sophora occurring in New Zealand (S. microphylla and S. tetraptera). This number, which may prove to be the commonest in the genus (5), could arise from a primitive stock with x = 8 by aneuploidy. For Canavalia
[Footnote] * New Zealand Genera.
[Footnote] * New Zealand Genera.
[Footnote] * Except for carmichaelia exsul in Lord Island.
a 2n number of 22 is established (5), but this genus may well be a recent immigrant to the New Zealand region since its only representative therein is a littoral species in the Kermadec Islands.
It thus appears that the ancestral Leguminosae of New Zealand, in accordance with Senn's speculation, had a basic number of x = 8.
III.—Taxonomy of the New Zealand Brooms
1. Phylogeny of the Group
Pod characters, inflorescence type, and general habit are the main criteria used by the taxonomist for distinguishing subgenera and genera. From these data and also the nature of the leaf trace systems, tabulated in Table III, a diagram (Fig. 2) has been formed to illustrate the possible phylogeny of the group.
Ancestral type I is supposed to have all features of a typical papilionaceous plant, being a round-stemmed shrub with relatively long, many-seeded pod and with the racemes borne singly or in clusters at the nodes. Its leaf trace system was of the primitive trilacunate form. From this ancestral form arose two main lines:
Those in which the round stem had been retained, and
Those in which the stem had commenced to flatten between the original lateral traces of two successive trilacunate nodes.
The first line accompanied by the reduction of pod size and the loss of all but one raceme at a node gave rise to Kirkiella. The ancestral type II on the second line, had all features of the ancestral type I except that flattening of its stem was accompanied by addition of flanking traces. From the ancestral type II arose the main “Carmichaelian line”. The character of one or many racemes borne at a node was maintained, pods were few-seeded and dehiscent, while there was also a retention of the flanking trace system and flattened stems. Monroella and Petriea exhibit these features and by the development of indehiscence of the pod, the Huttonella subgenus subsequently arose. Data of Table III are incorporated in Fig. 2 to illustrate further changes that have occurred as the other subgenera of Carmichaelia have evolved.
The “Notospartium line” is the second one arising from the ancestral type II. Indehiscence developed in a many-seeded pod and all racemes except one at a node were lost, as postulated in ancestral type III. The modern Notospartium genus has lost all spring or reversion shoots bearing laminate leaves, and the flanking trace system present in its seedling becomes modified in the adult plants (12).
The main feature distinguishing the genus Corallospartium from the subgenus Huttonella of Carmichaelia is the furrowed stems. Corallospartium crassicaule has fascicles instead of the clustered racemes present in Huttonella, but Corallospartium crassicaule var. racemosa retains these.
As proposed previously (12) Chordospartium seems at some time to have arisen by hybridisation between Corallospartium and Notospartium, and its features listed in Table III show that four of the main taxonomic criteria have been handed on by Corallospartium while two, that of general habit and a singly borne raceme, have been inherited from Notospartium.
2. Critical Discussion of the Taxonomic Status of Subgenera and Genera within the Group
The structure of the pod and its dehiscence are the main criteria used for distinguishing the genera and subgenera. The taxonomists have also utilised those features of general form and habit which are characteristic. Apparently in a few cases these morphological features have had greater significance attached to them. For example Cheeseman (3) says: “If differences of habit of inflorescences, etc.” (i.e., other morphological features) “are not considered sufficient to separate Corallospartium and Chordospartium, then by parity of reasoning similar characters of no greater importance cannot be used to distinguish both these groups from Huttonella of Carmichaelia and these three genera must merge into one. Some botanists must prefer this course, for it is very much a matter of taste and personal idiosyncrasy whichever view is adopted.”
In view of the fact that the indehiscent pod of Huttonella is present in Corallospartium and Chordospartium, let us examine the morphological features of these genera to see if there is sufficient evidence for them to be retained as distinct genera.
i. The furrowing of the stem is perhaps the most striking feature, but this has been shown (12) to result when there is excessive development of the fibrous caps of the leaf trace bundles. In the course of its development Corallospartium passed through a shallow grooved stage comparable with grooving of some Carmichaelia stems. Moreover, in the genus Celmisia of the Compositae, similar stomatal furrows occur in the leaves of a few species—e.g., Celmisia lyalli, but there has been no question of separating these from that genus on account of their furrowing. Furrows are apparently a xeromorphic feature which in the course of evolution has been selected under certain environmental conditions.
ii. The nodal systems of Corallospartium and Chordospartium are also closely related to those of Monroella, Petriea and Huttonella, in which extra traces flank the original lateral traces.
iii. Cheeseman attached considerable importance to the form of the inflorescence. However Corallospartium crassicaule var. racemosa, bearing either solitary or fascicled racemes, provides an intermediate type between Corallospartium crassicaule and Huttonella of Carmichaelia. One could conclude then that the morphology of the inflorescence is not a reliable criterion for taxonomic distinction of Corallospartium from Carmichaelia.
B. The subgenus Monroella of Carmichaelia
Data so far suggest that morphological differences alone are not really sufficient reason for Corallospartium and Chordospartium being distinct genera. However, if the taxonomist feels justified in keeping these genera distinct on account of morphological features, there is then one subgenus of Carmichaelia, Monroella, which has stems presenting the same degree of anatomical difference as those of Corallospartium and Chordospartium. The cortical fibre in Monroella has become scattered or “diffuse” in arrangement. This feature is constant in all three species of Monroella (Carmichaelia astoni, Carmichaelia monroi, Carmichaelia hollowayi),
C. The genus Notospartium
Notospartium, possessing a very distinct many seeded jointed pod, has attained taxonomic distinction from Carmichaelia. There are morphological features which also favour this isolation. The adult Notospartium species are the only brooms failing to develop laminate leaves or vestiges of these, either on the spring shoots or on” reversion” shoots (12). Corallospartium, Chordospartium and many species of Carmichaelia have laminate leaves varying from 1—7 foliate. Some members of the subgenus Carmichaeliella e.g., Carmichaelia aligera and Carmichaelia williamsii, although normally lacking these in the adult, do produce” reversion” shoots bearing foliate leaves, but the Notospartium species have lost all trace of this ancestral laminate leaf form except in their seedlings.
The leaf trace system of Notospartium differs from those present in the other genera (12). However the general histological and anatomical features do suggest that Notospartium and Carmichaelia at one time were more closely related.
The raceme of Notospartium is much longer than in any Carmichaelia species and resembles that present in Chordospartium. This feature raises the problem of connecting Chordospartium with the other genera. Its possible hybrid origin has been discussed (12). It seems more than coincidental that the raceme structure, general dimensions of the stem and habit of the stems of Notospartium, should have developed in Chordospartium along with furrows, the presence of unifoliate laminate leaves on the spring shoots, the leaf trace systems and the pod structure of Corallospartium. Until further investigation has been made it is impossible to know whether this is a relatively recent hybrid or one which has occurred earlier in the evolution of the group.
3. Species Problem and Hybridisation Within the Group
Simpson (13) acknowledges no conclusive evidence of hybrids between Carmichaelia species, but Cockayne and Allan (4) list two suspects Carmichaelia monroi × Carmichaelia subulata and Carmichaelia robusta × Carmichaelia subulata. Simpson says that the species Carmichaelia subulata is indeterminable. He says, however, “that many compound species do exist, and for the present it would be unwise to presuppose the existence of any one simple species.” I have seen specimens of Carmichaelia grandiflora collected from different localities and growing in Mr. Simpson's garden, and there is no doubt of the wide range of variability, the cause of which has not been studied.
Arising out of the review of the taxonomy of the group two suggestions are made:—
1. That Corallospartium and Chordospartium be included as two further subgenera of Carmichaelia. This is not a new idea, for Kirk (7) originally named Corallospartium, Carmichaelia crassicaulis.
2. That Notospartium should be retained as a distinct genus.
The writer is indebted to the New Zealand Federation of University Women for a Fellowship enabling her to carry out this work at the University of Cambridge. Thanks are also due to Dr. D. G. Catcheside, Dr. H. W. Howard, and
Metaphase plate from a root tip of Notospartiuom carmichacliae photographed at two levels of focus × 2,700
Fig. 1—Key-drawings to the two photohgraphs of Plate 1. Approx. × 1,000.
Mr. E. J. H. Corner for assistance in preparing this manuscript, and to several field-botanists and horticulturists who assisted in collecting seed for the chromosome analysis.
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11. Shaw, G. W. A Rapid Permanent Method for Chromosomes Nature, 165, p. 408, 1950.
12. Slade, B. F. Comparative Anatomy of the Cladode in New Zealand Blooms Trans. Roy. Soc. N.Z., vol. 80, Part I, p. 81, 1952.
13. Simpson, G. A Revision of the Genus Carmichaelia Trans. Roy. Soc. N. Z., 75, p. 231, 1946.
14. Thomas, P. T. The Aceto-carmine Method for Fruit Material. Stain Tech., XV, p. 167, 1940.