Ann. Bot. 45(180):639-645 (Oct 1931).
Species Hybrids in Aquilegia.
(Missouri Botanical Garden)
(John Innes Horticultural Institution.)
With four Figures in the Text.

FROM a morphological point of view most studies of species hybrids have been relatively unproductive. Mendelian analysis, though it has been of great assistance in the investigation of varietal differences, has been possible in only a few instances—East (4), Chittenden (1). Even in these favoured cases it has done little more than demonstrate that certain factors probably behave much the same way in hybrids as they do in the parent species.

In the following preliminary investigation of species crosses in Aquilegia, an attempt has been made to devise methods of morphological analysis which will on the one hand be useful in codifying the data from crosses between species, and which, on the other hand, will yield objective evidence as to the probable course of evolutionary development. This preliminary account is published to draw the attention of other workers to the problem in the hope that further methods may be developed.

For a general study of interspecific hybrids the genus Aquilegia is excellent material. Great morphological diversity between species is accompanied by unusual interfertility. The genus numbers roughly eighty to one hundred species, native to Europe, Asia, and North America. They exhibit remarkable morphological and ecological diversity, and can be grouped into at least five different sections. In spite of this diversity, every species cross which has been attempted has produced a first generation hybrid, and all of these have been at least partially fertile.

Morphologically the genus is particularly interesting by reason of its distinctive petals, which are sometimes interpreted as modified honey glands (6). Fig. 1 represents the petals of eight species drawn to scale. As shown in the figure, they are developed anteriorly into a broad blade or 'lamina', and prolonged below into a more or less narrow spur with the actual gland at its apex. In the widespread section Cyrtoplectrae (A. vulgaris, d; and A. pyrenaica, e), which includes species in Europe, Asia, and America, the lamina and spur are about equally developed, and the latter is more or less incurved. Related to this section are several curious Asiatic species (represented in the figure by A. viridflora, b; and A. fragrans, c) in which the lamina is much wider than long. In the living flower the wide laminae overlap and produce a peculiar cuplike effect. The American sections, Rhodanthe and Macroplectrae, are both characterized by long and narrow spurs. In the Macroplectrae (A. caerulea, f) the lamina is also well developed. In one member of this group, A. longissima, the spurs sometimes reach a length of 15 centimetres. In the section Rhodanthe, represented in the figure by A. canadensis (h) and A. Skinneri (g), the spurs are well developed and the laminae are short.

FIG. 1. Petals of various species of Aquilegia, in full face and in profile. (a) A. ecalcarata Maxim. (b) A. viridflora, Pall. (c) A. fragrans, Benth. (d) A. vulgaris, Lino. (e) A. pyrenaica, DC. (f) A. caerulea, James. (g) A. Skinneri, Hook. (h) A. canadensis, Linn. Natural size.

In the Chinese species, A. ecalcarata, Maxim. (Fig. 1, a), and a few other related forms, the spur is represented by a small pouchlike gland.

There are good reasons for believing that this represents the ancestral condition of the genus rather than a 'reduction'. In the first place it is similar to the honey glands at the bases of the petals in many of the related genera of the Ranunculaceae. In the second place, A. ecalcarata differs from the remaining species of the genus in several other characters, such as the shape of its fruit, the colour of its flowers, and its general aspect. In all of these characters it reminds one of certain genera closely related to Aquilegia, such as Isopyrum or Anemone. Drummond and Hutchinson (3) even transferred it to Makino's genus Semi-aquilegia, concerning which they wrote: 'Semi-aquilegia, with saccate petals, the only remaining link showing the origin of the remarkable genus Aquilegia, which as evidenced by the great variability of its species and its wide range, is probably still in a state of flux.'

1. See, for instance, Ulbrich (8).

Be that as it may,1 we undoubtedly do possess in the spur of Aquilegia a series of forms showing various degrees of similarity, which might easily be arranged in a phylogenetic sequence. But in Aquilegia such a speculative arrangement can be treated experimentally, for the various members of the series are interfertile and can be crossed one with another. Not only that, but in ecalcarata we even possess 'a living fossil', similar to the ancestral type in many respects, yet capable of producing fertile hybrids with its more highly elaborated sister species.

Cytologically, Aquilegia is not very favourable material. The chromosomes are very small; the pollen mother-cells are not easy to fix. The following numbers have previously been reported:

  n. 2n. Reported by
A. atropurpurea   14 Langlet
A. chrsyantha 7   Scalinska
A. haylodgensis, Hort.   14 Langlet
A. vulgaris 7   Winge
A. vulgaris parviflora   14 Langlet
A. vulgaris x chrysantha 7   Skalinska

The following root-tip counts were made:

A. flabellata, Hort. 14
A. chrysantha, Gray (New Mexico) 16
A. chrysantha, Gray (Arizona) 16
A. vulgaris,  var. 'China Blue' 162

2. The flabellata was of commercial origin. The specimens of chrysantha were grown from seed from wild stands. 'China Blue' is the laboratory name of a variety which turned up among the stocks originally got together at the John Innes Institution from various sources and handed over to the junior author in 1927. Its origin is unknown to us; it clearly belongs to A. vulgaris, L. and it breeds true. It is slow growing, about half the height of typical vulgaris but with flowers of the same size. The flowers are of a light-blue colour and tinged red in the bud. This very distinct variety has clearly originated from type vulgaris by some form of cytological aberration.

The chromosome numbers reported above indicate no gross chromosomal differences in the genus. Aquilegia is, therefore, one of those genera like Ribes, in which sizes, shapes, and numbers of the chromosomes remain relatively constant in spite of great external differentiation between the species.

The work reported below was begun independently by the senior and junior authors at the Missouri Botanical Garden and the John Innes Horticultural Institution respectively. In each case material of as many species as possible was obtained from seedsmen and botanic gardens. and a number of crosses were made. Experience has proved that practically all of this material is unreliable for precise work, and the study is being repeated with authentic material. The following crosses, however, were considered worthy of a preliminary examination, and it is with the results obtained from them that the present paper is concerned.

A. vulgaris compacta x A. formosa (1).
A. caerulea x A. vulgaris (2).
A. ecalcarata x A. chrysantha (3).
A. ecalcarata x A. vulgaris compacta (4).
A. chrysantha x A. vulgaris ‘China Blue' (5).
A. vulgaris x A. Jaeschkani (6).
A. vulgaris x A. chrysantha (7).
A. vulgaris x A. canadensis (8).

Certain regularities of interspecific dominance were apparent even in this material. They can be roughly summarized by saying that all of the first generation hybrids were more like Aquilegia vulgaris (in the broad sense) than might have been expected. This was true even when A. vulgaris was not one of the parents. Similar results have been obtained by other investigators. Cockerell (2), for instance, crossed two American species, A. chrysantha and A. desertorum. As regards the hybrid he wrote: the form of the flowers departs from both parents in the direction of the A. vulgaris group'. In none of the cases in the literature, however, were enough different crosses attempted to suggest that the 'vulgaris' appearance of the F1 was a general phenomenon. It maybe mentioned, however, that it is a common experience of those who grow Aquilegia as a garden flower that the vast majority of seedlings from natural seed have the appearance of vulgaris.

FIG. 2. Dissections of A. ecalcarata, A. 'chrysantha' (garden form), and A. vulgaris 'compacta' (garden form), with the F1 hybrids between them. The hybrids are all very much alike, and resemble vulgaris much more than they do either of the other two species involved.

Three of the most interesting crosses are illustrated in Fig. 2. The parents used were:

1. A. ecalcarata, Maxim. (with practically no spur).
2. Garden varieties of an American long-spurred species.
3. A garden variety of A. vulgaris (medium spurred).

In contrast to the great differences between the parents is the remarkable similarity of the hybrids. Theoretically we might have expected half-sister hybrids to show half the divergence of their unlike parents. That is to say, if we call the three species A, B, and C, and the F1 hybrid between A and B is represented as AB, and that between A and C as AC, and so on, we might have expected the following relations to hold:

A mere glance at Fig. 2 enough to show that this is clearly not the case. The three hybrids are very much like each other, in spite of the great differences between their parents. If we test the matter objectively, our opinion is adequately confirmed.

FIG. 3. Analysis of the ratio lamina length/lamina width in first generation hybrids. The vertical lines represent the difference between the parents in respect of this character. The arrow indicates the position of the F1 hybrid on the same scale.
1. A vulgaris compacta x A. formosa.
2. A caerulea x A. vulgaris.
3. A. ecalcarata x A. chrysantha.
4. A. ecalcarata x A. vulgaris compacta.
5. A. chrysantha x A. vulgaris 'China blue'.
6. A. vulgaris. x A. Jaeschkani.
7. A. vulgaris. x A. chrysantha.
8. A. vulgaris. x A. canadensis.

Not only are these three hybrids much more alike than we might reasonably have expected, but they are more like A. vulgaris than they are like any of the other species involved. This is equally true of the other crosses studied. An attempt was made to measure the resemblances more exactly, and the results are presented in Figs. 3 and 4. Two characters were chosen for study, spur length and lamina length. In order to remove the effects of hybrid vigour, in so far as they cause changes in size, comparative ratios were used. Since the width of the lamina at the orifice is a relatively constant character it was used as a basis for comparison, the actual ratios studied being:

1. Length of lamina/width of lamina.
2. Length of spur/width of lamina.

Figs. 3 and 4 represent for each of the eight crosses the position of the parent species on the two scales, the theoretical intermediate positions for the first generation hybrid, and its actual position (at the right).

The tables present in a more exact way what has already been stated in general terms. In every case where the hybrid is not at the theoretical midpoint between the parent species, the shift is in the direction of medium length spurs and wide laminae. In other words, in species crosses in Aquilegia there is a general dominance of medium length spurs over very long or very short spurs, and of wide laminae over long laminae. This is without any evident relation to the particular species used, and is irrespective of the direction in which the crosses are made.

The regularity of the phenomenon seems to be adequately established even by these preliminary studies. For its accurate interpretation further work with more authentic material will be necessary. Were it not for the results obtained with A. ecalcarata one might ascribe the phenomenon to recombination, such, for instance, as is responsible for 'atavism' in crosses between breeds of domestic animals. But A. ecalcarata, though certainly more 'primitive' in its characters than A. vulgaris, is quite as recessive to A. vulgaris as are the 'advanced' American long-spurred species.

FIG. 4. Similar to Fig. 3, but giving the ratio spur length/lamina width.

It is not impossible that the whole problem is a more purely physiological one than might have been expected. It may be that the nuclear substances which produce medium spurs and wide laminae are more effective when diluted than those which produce long spurs and long laminae. In that case we are down to the physiological growth relationships which have limited in a broad way the directions of evolution within the genus.

The authors wish to thank Miss Irma Andersson-Kottö, who provided the nucleus of the material in 1927.

Part of the work reported upon in the above paper was performed while the senior author was a fellow of the National Research Council, to which due acknowledgement is made.


Preliminary studies indicate that species hybrids in the genus Aquilegia tend to resemble A. vulgaris, even where that species is not one of the parents.


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