The Mechanism of Creative Evolution
C C Hurst, 1932

Chapter IIX

Many of our useful and beautiful garden plants belong to this category, their extra size, compared with the wild forms from which they arose, being the result of their carying quadruple or tetraploid sets of chromosomes instead of double or diploid sets. Some of our garden roses are of ths type, also many flowering bulbs as well as various fruits and vegetables. When the chromosomes are investigated another point comes to light. When the germ-cells are formed the chromosomes, instead of forming up in pairs in the usual way, often form groups of four, the four like chromosomes going together. Fig. 76 d shows this in a duplicated form of the Himalayan rose (Rosa macrophylla) known as var. Korolkowii. This was found growing in a garden in Khiva and is identical with the parent species except for the extremely large size of its parts. In the illustration of the germ-cells it will be seen that the wild plant with the normal double set of 7 chromosomes (14) forms seven pairs while the garden variety has in this particular cell six groups of four chromosomes each and two pairs, one group of four having failed to come together.

Fig. 76. Tetraploid variety in Rosa.

On the left, chromosomes, flower and fruit of the diploid species R. macrophylla Lindl.; on the right, the tetraploid variety Korolkowii.

The diploid has, a, 14 somatic chromosomes and, b, 7 pairs (bivalents) in the germ cells, while the tetraploid has, c, 28 somatic chromosomes and, d, its germinal chromosomes are often in fours (quadrivalents), the figure showing 6 quadrivalents and 2 bivalents.

Rosa macrophylla Lindl. Botanische wandplaten, p. xxx () [J. Vuijk]

Chapter IX

A diploid species has two sets of chromosomes and genes, one derived from each parent. A polyploid species, in common with a polyploid variety, has multiple sets of chromosomes and genes, tetraploids with four sets, pentaploids with five, hexaploids with six, octoploids with eight sets, and so on. Polyploid species, however, differ fundamentally from polyploid varieties, inasmuch as their polyploid sets of chromosomes and genes are those of more than one diploid species while polyploid varieties have the chromosomes and genes of a single diploid species. In other words, polyploid varieties carry the same specific genes in a multiplex state with multivalent chromosomes, while polyploid species carry the genes of more than species in a duplex state with bivalent chromosomes. If A and B represent the differential sets of chromosomes and genes of two diploid species AA and BB, then AAAA is a tetraploid variety of the diploid species AA, while AABB is a tetraploid species combining the chromosomes and genes of the two diploid species AA and BB The most widely investigated case of polyploid species is that of the genus Rosa (Täckholm., 1922; Hurst, 1928). More than one thousand species and varieties of this genus have been studied cytologically, taxonomically and genetically by nine workers in Europe and America and all the known species of this polymorphic genus have been investigated. Roses are very widespread throughout the northern hemisphere reaching nearly to the equator and extending beyond the arctic circle. Naturally with such a wide distribution there are many different species ranging from the giant diploid Rose (Rosa gigantea Coll.) which climbs the trees of the Burmese forests and which with its long orange-cream flower buds and huge ivory coloured flowers is one of the glories of the plant world to the small and insignificant polyploid species only a few inches high which grow on the Canadian prairies and in the tundra and arctic regions of Europe, Asia and America. When the chromosomes of all the different species were examined it was found that though the number was usually constant for each wild species there were several different numbers for the different species of the genus. Species were found with 14, 28, 35, 42 and 56 chromosomes and hybrids with 21. It will be observed that these numbers are all multiples of 7 so that the genus consists of species which are diploid and polyploid since all bear chromosome numbers which are multiples of the basic gametic number of the genus. Thus there are diploids with 14 chromosomes triploids with 21 tetraploids with 28 pentaploids with 35 hexaploids with 42 and octoploids with 56 chromosomes. The diploid and equal polyploid species with balanced bivalent chromosomes form regular germ-cells and pollen grains with a normal meiosis and gametogenesis which does not differ materially from that of other species of plants and animals. The unequal polyploid species with unbalanced (bivalent and univalent) chromosomes consisting of 7 bivalents and either 7, 14, 21 or 28 univalents have an irregular meiosis followed by a regular but unequal gametogenesis in which the female gametes have twice, three times or four times the number of chromosomes found in the functional male gametes. The pollen grains of these species are polysporic and only those with 7 chromosomes are functional. This remarkable process of gamete formation so far appears to be unique in plants and animals. The triploids with 7 bivalents and 7 univalents are all hybrids or cultivated varieties which are sterile and apparently cannot persist as species in a wild state.

To the pentaploid species belong the wild Dog Roses and briars of our hedges (R. canina L.). These having unbalanced sets of chromosomes do not form regular pairs but there are 7 paired bivalents and 21 single univalents in their early germ-cells, male and female. In the pollen mother cells the 7 pairs divide first and irregular pollen grains are formed in which some have only the 7 chromosomes while others contain the 7 with a variable number of the univalents and others again carry only univalents. As a rule only those with the 7 from the bivalents behave normally and produce functional pollen, the end result being that the 21 univalents are discarded and take no part in the male heritage. In the embryo-sac mother cells a remarkable phenomenon appears. All the 21 univalents go to the top of the cell and wait there for the reduced 7 of the bivalents. These together form a cell with 28 chromosomes and this develops to form an embryo-sac containing the egg-cell with 28 chromosomes (fig. 97). Thus we have the egg-cells with 28 and the male nuclei with 7 chromosomes, and when they come together at fertilisation they make a new pentaploid embryo and individual with 35 chromosomes like the parent, in this way completing the cycle of the most remarkable mechanism yet discovered in the formation of gametes.

Fig. 97. A comparison of the formation of male and female gametes
in the unequal pentaploid species of Rosa.

(a-c) pollen mother cells with 7 paired chromosomes which work independently of the 21 single chromosomes, giving male gametes with 7 chromosomes only.

(d-f) corresponding divisions in embryo-sac mother cells with regular but unequal reduction of the chromosomes, giving embryo-sacs with 28 chromosomes.

(a) 1st pollen mother cell division metaphase in R. glaucophylla Winch, var. Seringei (Christ), 7 paired chromosomes in centre, 21 single chromosomes around

(b) 1st pollen mother cell in R. uriensis Lag. et Pug., 7 paired chromosomes reduced, 7 to each pole, the singles splitting

(c) 2nd pollen mother cell division in R. orophila Gren., 7 of the paired chromosomes at each pole to make pollen grains

(d) 1st division embryo-sac mother cell metaphase in R. glaucophylla Winch, 7 paired chromosomes on spindle, 21 singles at upper pole

(e) 1st embryo-sac mother cell division in R. elliptica Tausch, 7 chromosomes below, 28 above (7 from reduced pairs + 21 singles)

(f) 2nd embryo-sac mother cell division in R. Froebelii Christ, 7 chromosomes in lower cell, 28 in upper (latter forming the embryo-sac)

The unequal tetraploid species with 7 bivalents and 14 univalents and the unequal hexaploid species with 7 bivalents and 28 univalents follow the same mechanism in their gamete formation.

When the taxonomic characters of all the polyploid species were examined and analysed it was found that the wild polyploid species were not, as originally assumed, merely polyploid varieties of the diploid species which had reduplicated their chromosomes, but that they were composed of the characters and chromosomes of several diploid species combined together (Hurst, 1924, 1925). That is to say, in the equal tetraploid species were found the characters and chromosomes of two distinct diploid species, in the equal hexaploid those of three species and in the octoploids the characters and chromosomes of four distinct diploid species. In fig. 98 we see an illustration of this in the tetraploid species Rosa spinosissima L. found on the sand dunes of Western Europe. This rose is tetraploid in its chromosomes (28) and an analysis of its taxonomic characters shows that it is composed of the characters of two distinct diploid species represented in the figure by the two Linnean species R.  rugosa Thunb. and R. Willmottiae Hemsl., each of which has 14 chromosomes. The figure shows that the tetraploid species R.  spinosissima combines the tortuous, excessively prickly branches of rugosa with the singly set flowers and small leaflets of Willmottiae, and so on with the other specific characters. Genetical experiments at Cambridge confirm the taxonomic analyses in so far as hybrids between similar Linnean species to the two above present the salient taxonomic characters of R. spinosissima.

A comprehensive cytological and taxonomic investigation of all the Linnean species of Rosa L., confirmed by many genetical experiments, shows that in Rosa L. proper, there are five basic diploid genetical species, each with a differential gametic septets of chromosomes and genes (genome) distinguished as A, B, C, D and E, thus constituting five diploid septet species with somatic chromosomes and genes AA, BB, CC, DD and EE respectively (fig. 99). Each of the five genetical species includes a number of closely related Linnean species which by reason of their complete fertility when inter-crossed, and their regional geographical distribution, have been classed as genetical subspecies (Hurst, 1928).

Fig. 98. R. spinosissima and its parents

Above, types of the two diploid species of which it is composed, showing the characters which appear in it:

(a) branching habit of the CC diploid species R. rugosa Thunb.
(b) flowering and leaf habit of the BB diploid species R. Willmottiae Hemsl.






Middle, the tetraploid BBCC septet species Rosa spinosissima L.

(c) branching
(d) flowering




Below, chromosomes of the three species

(e) R. rugosa diakinesis with 7 paired chromosomes
(f) pollen grain of R. spinosissima with the reduced gametic number (14 chromosomes)
(g) R. Willmottiae with 7 paired chromosomes

Fig. 99. Representative Linnean species (genetical sub-species) of the five basic diploid genetical species
of Rosa L. together with their chromosomes in different stages.

AA septet species R. arvensis Huds. with 7 bivalent chromosomes in pollen mother cell

BB septet species R. Webbiana Wall. with 7 bivalents in embryo-sac mother cell

CC septet species R. coruscans Waitz. with 7 bivalents in pollen mother cell

DD septet species R. Fendleri Crép. end of pollen mother cell divisions, showing the reduced number of chromosomes, 7 at each pole

EE septet species R. macrophylla Doncasteri with 7 bivalents in pollen mother cell.

All the wild polyploid species of Rosa L. are composed of various combinations of these five diploid septet species. On this basis 26 polyploid septet species with balanced bivalent chromosomes are possible, viz. 10 tetraploids, 10 hexaploids, 5 octoploids and 1 decaploid and of these 21 have so far been identified in nature, viz. 10 tetraploids, 8 hexaploids and 3 octoploids. With a few exceptions each of these polyploid genetical species corresponds with the Linnean species and all have balanced bivalent chromosomes with regular and normal meioses and gametogeneses.

The tetraploid species R. spinosissima L. shown in fig. 98 carries the septets BBCC, while the diploid Linnean species R. Willmottiae Hemsl. carries BB and the diploid R. rugosa Thunb. carries CC septets, the combination of the chromosomes and genes of the two diploids BB and CC making the tetraploid species BBCC.

The interactions of the differential septet genes in the polyploid species prove to be an interesting study. Certain septet characters usually predominate over the corresponding characters of the other septets, e.g. the exserted styles of the A septet; the singly set flowers of the B septet; the glandular acicles of the C septet; the short straight woolly styles of the D septet; and the rimmed pendulous fruits of the E septet.

Fig. 100 Illustrates the predominance of the singly set flowers in tetraploid, hexaploid and octoploid species carrying the septets BB, not withstanding that in the last two species the B septets are in a definite minority. Fig. 101 shows the predominance of the rimmed pendulous fruits of the E septet.

Fig. 100. Illustrating the predominance of the B septet character, singly-set flowers, in several
polyploid Linnean species which carry the B septet of chromosomes.

(a) Rosa Hugonis Hemsl. BB diploid species with 7 bivalent chromosomes in pollen mother cell

(b) R. ochroleuca Swartz BBDD tetraploid species with pollen grain carrying 14 chromosomes

(c) R. spinosissima L. var. duplex Hort. BBCC tetraploid species with 28 somatic chromosomes in body-cell

(d) R. Moyesii Hemsl. et Wils. AABBEE hexaploid species with 21 bivalent chromosomes in pollen mother cell

(e) x R. hibernica Templ. ABBCDD hexaploid natural hybrid with 14 bivalent and 14 univalent chromosomes in pollen mother cell

(f) R. altaica Willd. BBDD tetraploid species with 14 gametic chromosomes in pollen mother cell

(g) R. acicularis Lindl. p.p. BBCCDDEE octoploid species with 28 gametic chromosomes in pollen mother cell.

Fig. 101. Showing the predominance of the E septet character, type of fruit, in Rosa polyploids.
(1, 2) fruits of the EE diploid species R. macrophylla Lindl.
(3) R. Fargesii Hort. (tetraploid AAEE)

(4) R. Davidii Crép. (another Linnean tetraploid AAEE)
(5) R. Moyesii Hemsl. and Wils. (hexaploid AABBEE)

(6) R. acicularis Lindl. p.p. (octoploid BBCCDDEE)

With regard to the unequal polyploid species with unbalanced bivalent and univalent chromosomes: omitting those with more than one septet of bivalents which are natural hybrids and omitting triploids which are not found in a wild state as established species, 55 polyploid septet species are possible, viz. 30 tetraploids, 20 pentaploids and 5 hexaploids; of these 20 have so far been identified in nature, viz. 4 tetraploids, 13 pentaploids and 3 hexaploids. With a few exceptions these also correspond with Linnean species and all have 7 bivalent chromosomes together with 14, 21, or 28 univalents, giving rise to irregular meioses followed by regular but unequal gametogeneses. The cytological evidence supports the hypothesis of Täckholm (1922) that these unequal polyploid species with unbalanced chromosomes are ancient Fl hybrids and the taxonomic and the genetical evidence (Hurst, 1928) fully confirm this hypothesis. According to the geological evidence these unequal polyploid species arose about the middle of the Pleistocene Period after the great Mindel glaciation, and it is a remarkable fact that the present distribution of these species in Europe and Western Asia corresponds rather closely with the area influenced by the advance and retreat of the Mindel ice sheet. The advance of the ice would naturally bring down the surviving arctic polyploid species to mingle with the southern diploid species, thus producing the unequal polyploid F1 hybrids which have since that time reproduced themselves true to type, mainly by apomictical reproduction. It is interesting to note that the lowland and southern unequal polyploid species R. canina L. and R. micrantha Sm. carry the bivalent septets AA which are also carried by the lowland and southern diploid species R. arvensis Huds., while, on the other hand, the alpine and northern unequal polyploid species R. caesia Sm. (R. coriifolia Fries) and R. glaucophylla Winch (R. glauca Vill.) carry the bivalent septets DD which are also carried by the alpine and northern diploid species R. cinnamomea L. 1759.

How the equal polyploid species with balanced bivalent chromosomes arose is an interesting problem which may ultimately have several solutions. At present the most feasible and acceptable hypothesis is that the original diploid species of the Miocene and Pliocene were hybridised by insects and that the chromosomes of the hybrids were duplicated, thus producing fertile polyploid species. Genetical experiments confirm this hypothesis so far as the characters of the diploid and polyploid species are concerned, but as yet duplication of the chromosomes in hybrid roses has not been observed under experimental conditions. [CybeRose note: R. Kordesii and Bayse's Amphidiploid are more recent.] This may, however, be due to the recognised technical difficulties peculiar to the sexual reproduction of woody shrubs, since in genetical experiments with herbaceous genera at least seventeen cases are definitely known where two species of plants have been hybridised and a subsequent duplication of the chromosomes has produced new fertile polyploid indiviuals which in every respect satisfy the taxonomical requirements of a new species. These individuals constitute an entirely new species, inasmuch as in combining the characters of two distinct species a new end result is achieved which is distinct from either of the parent species. Further, the increased number of chromosomes isolates them sexually from their parents, since though quite fertile with one another they produce sterile triploids back-crossed with either parent. In this way new species and in a few cases new genera have arisen under experimental control, and there can be no doubt that the seventeen recorded cases provide an experimental demonstration of the evolution of new species.

[deletions not related to Rosa and close kin]

It is an interesting fact that in Rosa the species in the extreme south towards the equator are all diploids, while the polyploids have a more northerly distribution, the octoploid species R. acicularis Lindl., which is the highest polyploid species yet found in this genus, being, so far as critically known, usually arctic and circumpolar in its distribution, though from living material collected for me by Prof. Cockerell it apparently extends south to Lake Baikal in Siberia, where conditions are sub-arctic.


Other genera of the family Rosaceae besides Rosa have multiple numbers. Rubus (Raspberries and Blackberries) has precisely the same numbers as Rosa, the basic number being 7. In this genus also there are diploids with 14, triploids with 21, tetraploids with 28 pentaploids with 35, hexaploids with 42 and octoploids with 56 chromosomes. In Potentilla (Cinquefoils) diploids with 14 and tetraploids with 28 chromosomes have been found. Fragaria (Strawberries) also has septets of chromosomes in this case diploids with 14, hexaploids with 42, and octoploids with 56 having been found. In Prunus (Plums and Cherries) however, the basic number changes to 8 while in Alchemilla (Lady's Mantle) the lowest number yet found is 16 (gametic), with "tetraploid" species with 64 chromosomes. If 8 is also the basic number in this genus the diploids have been washed out of the genus may have originated from an old tetraploid which had come to resemble a diploid species by mutations and segmental interchanges between the chromosomes of the two sets.

The Pomoideae sub-family of the Rosaceae, including Pyrus (Apples, Pears, White Beam, Mountain Ash, and Service Tree), Crataegus (Hawthorns), Cotoneaster, Pyracantha, Cydonia (Quince), Mespilus (Medlar), Amelanchier and other less known genera, are remarkable in having 17 as their gametic number of chromosomes. Recent work by Darlington and Moffett on these genera has shown conclusively that this unusual number has arisen from ancestral species with 7 chromosomes. They find in their cytological preparations that of the 34 chromosomes present in the "diploid" 4 of the chromosomes are represented four times and 3 chromosomes are represented six times, since, when the gametes are being formed, instead of finding 17 paired chromosomes as expected, there are frequently different numbers of multivalent chromosomes in secondary association and in extreme cases there are four groups containing 4 chromosomes in each and three groups containing 6, making only seven groups in all. Thus these Pomoideae are "trebly hexasomic tetraploids", and the number 17 is a secondary basic number, having arisen by the duplication of the original 7 chromosomes and the subsequent reduplication, probably by non-disjunction, of three more pairs of chromosomes. The polyploids arising from it are termed secondary polyploids. This is an especially interesting demonstration of the origin of a new number of chromosomes and, judging from the effect upon the characters of the extra chromosomes in the transmutants that have arisen in cultures with one or more additional chromosomes (p. 82), it is highly probable that this subfamily owes its different fruit structure (pome) and other peculiarities, which distinguish it from the rest of the family, to its extra chromosomes, showing a new family in process of evolution. Those members of the Rosaceae with 8 or 16 chromosomes may have branched off by non-disjunction from the original 7 chromosome ancestors, which number still persists, as we have seen, in a large part of the family.


On the other hand we have the interesting case of Rosa acicularis, which is said by different cytologists to have diploid, tetraploid, hexaploid and octoploid forms. A critical taxonomic diagnosis of the material used shows clearly, however, that the original R. acicularis of Lindley is octoploid and includes two distinct genetical species with septets AACCDDEE and BBCCDDEE, while the American hexaploid "acicularis" are either R. Bourgeauiana Crép. which is BBCCDD, or R. Sayi Schwein., which is CCDDEE. The tetraploid "acicularis nipponensis" of Willmott (non Crép.) found in the Kew collection is a subspecies of R. pendulina L., which is DDEE, while the original diploid R. acicularis nipponensis of Crépin is a subspecies of R. rugosa Thunb., which is CC.

Hurst Bibliography