Introduction

C. C. Hurst observed that five large sets of characters (50), involving many parts of the plants, are found together in plants from around the world. Polyploids combine these sets of traits, some unit characters being dominant, recessive or intermediate in expression. In the mid-1920s, when Hurst began publishing his observations, the only way he could see for these traits to be transmitted together would be for the chromosomes of each diploid genome to be passed along as a group. This sort of behavior had been observed in evening primroses (Oenothera spp.), and works out very neatly for many small grains (wheat, rye, barley, etc.).

Hurst raised many hybrids, and observed the same combinations of characters found in naturally occurring species. For example, a cross between Rosa rugosa and R. willmottiae, though diploid, approximated the characteristics of the tetraploid R. spinosissima.

*"...we have reason to believe that the chromosomes of the father plant
and mother plant side by side represent blocks of parental characters..."

Interpersonal relationships are not easy to understand. Personal animosities, professional jealousies and petty rivalries can taint scientific discussions as they do many other endeavors. In this case, C. D. Darlington had a dislike for William Bateson, and was not averse to lying about him. E.g., Darlington claimed that Bateson did not believe in chromosomes, despite the fact that Bateson discussed chromosomes in a report* published the year before Darlington was born. Bateson and Darlington disagreed on some points, so Darlington took the low road. Bateson and C. C. Hurst were good friends, so Darlington disliked Hurst as well.

There seems also to have been some professional envy. Wikipedia provide an excellent example.

"Here is Hurst's concept of species in Creative Evolution (1932), p. 66-7:

"A species is a group of individuals of common descent, with certain constant specific characters in common which are represented in the nucleus of each cell by constant and characteristic sets of chromosomes carrying homozygous specific genes, causing as a rule intra-fertility and inter-sterility. On this view the species is no longer an arbitrary conception convenient to the taxonomist, a mere new name or label, but rather a real specific entity which can be experimentally demonstrated genetically and cytologically. Once the true nature of species is realised and recognised in terms of genes and chromosomes, the way is open to trace its evolution and origin, and the genetical species becomes a measurable and experimental unit of evolution."

"Such views were typical of the stance in evolutionary biology, adopted later by and today mainly credited to Theodosius Dobzhansky and Ernst Mayr, and dubbed "The Modern Synthesis" by Julian Huxley in 1942."

Darlington was among that group.

Treasure Your Exceptions: The Science and Life of William Bateson (2008) p. 655
Alan Cock, Donald R. Forsdyke

During the Second World War Rona Hurst joined the staff of Christ Hospital School where she taught for eighteen years. She wrote an elementary introduction to Genetics [25], in which she acknowledged help from Darlington. During the 1960s, she wrote The Evolution of Genetics, but could not find a compliant publisher. It was updated in 1974 with material from letters that Alan Cock had found in the Coleman microfilm (see Prologue). Darlington wrote to Alan (Oct. 10. 1973): "I think Mrs. Hurst's book is one I have warned publishers not to take: the widow of a long dead charlatan is something of a hazard." Rona got her own back in a letter to Alan (Feb. 10, 1980): "Darlington is an old sinner and I think you were wise to ignore him — my husband always did, he wastes too much time on his opponents ... and Julian Huxley was another. So far as I know neither have made any or very few real genetical experiments, and yet they have the cheek to question those who have!" She went on to relate the trouble she had had when she was assigned the role of "collaborator" with Darlington who was in charge of the National Rose Collection project at Bayfordbury, and "didn't believe in any of our rose work."

But did Darlington really not believe? C. C. Hurst died in 1947. Just two years later Darlington published the following paper, which applies as well to Hurst's observations for Rosa as it does for Rubus.


Crane & Thomas: Reproductive Versatility in Rubus (1949)
Heredity 3:103-106 (1949)
On An Integrated Species Difference
C. D. Darlington

The families examined by Crane and Thomas from the cross of Veitchberry by raspberry fall into two classes, the sexual and the asexual [parthenogenic]. If these classes are separated and compared two apparently contradictory conclusions follow.

In the first place there is a segregation of species differences in the diploid asexual progeny which is evidently genuine. But the three types of diploid are those expected only if the raspberry-blackberry difference is behaving as a unit in inheritance. Now this difference, as the diagram shows, is an elaborate one. It must obviously depend on numerous mutually adapted gene changes. The recombination of these in diploids is prevented in the European flora, both wild and cultivated, by the elimination of all diploid hybrids. We now see that it is also prevented in polyploids by some other, even more fundamental, condition. This condition must be the integration of all the differences as a block within which no crossing-over occurs.

  Diagram to show the contrasted effects of segregation on the raspberry-blackberry difference in sexual and asexual families of the hybrid.  
BLACKBERRY BB
R. rusticanus

Permanent brambles, unlimited growth
Few broad thorns
Palmate leaves
Black fruits—fixed plug
X






RASPBERRY RR
R. idaeus

Short-lived canes of limited growth
Many narrow prickles
Pinate leaves
Red fruits—free plug
 








X





* Genotypic failure of pairing at meiosis in pollen mother cells. VEITCHBERRY RR BB*

Intermediate in:
stems, prickles and leaves;
colour and flavour of fruit;
freedom of plug









Breeds true except for
intra-group differences
Asexual
4 BB infertile
2 RB* infertile
1 RR fertile
  Sexual
237 RRB

In the second place the eggs of the Veitchberry in sexual families (selfed as well as crossed) show no segregation of the differences between its parental species. If such segregation occurs the homozygotes are eliminated. The eggs in asexual families on the contrary show, not merely segregation, but an excess of homozygotes: it is the heterozygotes that are eliminated.

This apparent contradiction is resolved by the cytological evidence. The frequency with which any one chromosome of the seven types enters into a quadrivalent in the Veitchberry is 0.39/7 or 5.5% of the cells. A quadrivalent will give RR—BB segregation in about half these cells and of them half will suffer loss of laggards and die, leaving 0.7% of RR gametes and 0.7% of BB, or one in 140.

Thus one in 1402, or about 20,000, selfed seedlings of Veitchberry should show segregation of each of the pure types, a possibility which has not been tested.

On the other hand the 0.7% segregation will be enhanced in the asexual families by the elimination of the diploid homozygotes. This elimination is independantly shown by the failure of crosses between the diploid species—except in the production of polyploids such as the Veitchberry and the Mahdi. Thus the diploid elimination conceals the heterozygotes and the tetraploid recombination conceals the homozygotes.

The physiological integration of the raspberry and blackberry types in the Rubus cell must be combined with the basis of genetic isolation between the two groups. This isolation appears to be twofold: it is shown by the non-viability of direct diploid hybrids and also by the sterility of the indirect diploid arising from parthenogenesis of the tetraploid. This sterility strongly expressed appears as morphological male-sterility, and weakly expressed as reduced pairing at meiosis; in all gradations it is genotypically controlled.

The non-viability of the diploid hybrid is also clarified by the present experiment. It seems not to be inherent since the diploid RB heterozygote can be raised in this indirect way. It probably depends on an error in the embryo-endosperm-ovary relationship in the diploid first cross which is no longer completely effective as a means of eliminating the diploid progeny of the tetraploid hybrid.

One consequence of this unitary difference is in the interpretation of the Mahdi sport. The pentaploid has evidently arisen by doubling (vegetative or sexual) in the triploid followed by the loss of one chromosome set, i.e. seven complementary chromosomes taken at random from the 42.

The maintenance of the same external form with such a change of chromosome complement was preposterous in terms of a dispersed difference: it becomes intelligible in terms of an integrated one. To suppose that the 2 to 1 raspberry-blackberry proportion of the original Mahdi might persist in its giant mutant as a 3 to 2 proportion, avoiding any external qualitative variation by a precise regulation for all seven chromosomes of the set was too much. But to suppose that such a change could take place for a single member of the set although astonishing, is not utterly unreasonable. It now therefore becomes necessary to suppose that extensive changes hitherto known only in the "vegetative regulation" of the mosses (Wettstein, 1924) may take place in mitotic chromosome numbers, subject to mechanical loss of chromosomes and physiological selection of cells.

How is the raspberry-blackberry difference constituted?

In Oenothera we know how interchange makes it possible to hold together in the interchange hybrids large or complex differences lying in the seven different chromosomes. But here there is no evidence of interchange. All the differences must therefore lie in one chromosome.

We have to visualise a single super-gene consisting of many parts with mutually adjusted effects. These parts must be recombinable within each group as has been shown both for the prickles of the blackberry by Crane and Darlington (1927) and of the raspberry by Lewis (1940). It is in this way that an enormous range of species has been produced in each group, limited in number indeed only by the supply of materials and names that will enable us to preserve and describe them. On the other hand in crosses between the two groups the ultimate differences are held together. The diploid blackberry segregates arising from the Veitchberry have not merely the general blackberry character; they have the very prickles of their rusticanus grandparent.

Such a system can arise by the inversion of a chromosome segment. In most natural populations there are inversions floating which occasionally will chance to include (or, shall we say, collide with) groups of mutually adapted genes capable of becoming a focus of fruitful discontinuity. And the coincidence having occurred, the resulting super-gene will evolve in the way that has been described from its successive stages by Darlington and Mather (1949).

Thus, in a variety of ways, we have evidence of the basis of the chief genetic divarication within the genus Rubus. The fact that such an elaborate divarication is effective only at the diploid level shows that it is at this level that the long-term evolutionary processes have been at work, the polyploids being a very recent, indeed perhaps a post-glacial, novelty.

In the present super-gene difference we can already see two stages of development. The integration of a block by suppression of crossing-over, and the non-viability of the diploid hybrid, both of which prevent recombination. But the integration of the block must have come first since the non-viability of the hybrid between two groups automatically brings to an end this organised divergence.

The closest analogy to the present situation is in the speltoid and fatuoid complexes of the cereals. The difference is that these complexes have arisen in cultivated plants and in hexaploids. Moreover they distinguish smaller systematic groups and lead to no inviability in the hybrid. The Rubus super-gene is therefore presumably a much older complex.

The problem that now arises is thus to compare the character and scope of the discontinuity between different sections of the main raspberry and blackberry groups in different parts of the world.

References


There is also evidence that this "integrated species difference" involves a single chromosome in Rosa. For example, Austin (1993) wrote:

"The third line we pursued was by way of the Rugosa hybrid 'Conrad Ferdinand Meyer.' At first we harbored no great hopes of success, for we feared that the resulting seedlings from a cross with this excessively vigorous hybrid would be altogether too gross in character. 'Conrad Ferdinand Meyer' was itself a cross between the very popular and beautiful Climbing Noisette Rose 'Gloire de Dijon,' and an unknown Rugosa hybrid. It also had one of the most powerful and delicious fragrances. As before, we crossed with some of our better English Rose in particular 'Chaucer,' and had one of those pieces of luck that sometimes turn up in rose breeding. Some of the seedlings from this cross were of typical rugosa appearance, while others bore absolutely no resemblance to a Rugosa Rose. It seemed that some of our hybrids had taken the genes only from the 'Gloire de Dijon' half of 'Conrad Ferdinand Meyer,' while others had inherited those from the Rugosa side. What we had in many instances were in effect hybrids of 'Gloire de Dijon'."

In other words, the Rugosa character was transmitted as a single unit.

Buck (1960) had similar results when breeding with hybrids of Rosa laxa. Then there was Gustafsson (1944) who raised an odd Rugosa hybrid that differed dramatically from its sibling.

R. canina II x rugosa: 1934-4 This series consists of two individuals. One is a monosomic plant showing almost no trace of the father but also being very unlike the mother. It is smaller and less vigorous than its sister-plant, having few and rather weak prickles, small purely white petals, a light-green colour on stem and leaves, and lacks anthocyanin. It is less winter-hardy than its sister-plant; during the cold winters 1941 and 1942 the superterrestrial parts were entirely killed. It cannot be a monosomic of the mother-plant since the meiotic behaviour is that typical of canina-rugosa hybrids. Therefore in this case the loss of one chromosome obliterates the characters brought in by the rugosa genome.

The strange case of 'Margaret McGredy' producing a Rugosa-like sport suggests that whatever is controling the Rugosa-character-unit (for want of a better name) may be present but recessive, and then reawaken.

Hurst Bibliography