On the Origin of Hop Varieties by Means of Artificial Selection

Or the Preservation of Favoured Hop Varieties in the Struggle for Life.
My latest beer obsession has been how there came to be ten varieties of Goldings hops in the National Hop Collection. To explain how this came to be I shall not only have to bore you about hop history, but plant physiology and selection methods as well. No whimpering at the back there.
Hops are in the main dioecous, having separate male and female plants. As nothing ever seems to be totally clear cut in biology, occasionally monoecious plants will grow from seeds with flowers of both sexes, though in this case either the male or female flowers are often infertile (1).

It is overwhelmingly the female plants that are of interest to brewers, as it is the hop cones from the female plants that are used in making beer. In some countries it's even illegal to grow male hops.

Outside of breeding programmes hops are grown from cuttings. Even in England, where male hops are grown alongside the females at the rate of approximately 1:400, this is because it helps protect them from mildew and is not because the seeds are used for growing new plants.

The reason for this is that there is great genetic variation in hops, and seedlings are very seldom like the parents. Ian Hornsey (2) writes:"Because the sexes are separate the species is highly heterozygous with the resulting extreme morphological variability of plants that are raised from seeds. This causes problems for the commercial grower who strive for consistency. For this reason, therefore, a commercial grower will propagate the crop vegetatively, there being three major methods:

1. from hardwood cuttings;


2. from growing shoots that have been cut up and planted in sterile peat (called mist propagation);


3. by 'layering', This latter process involves the laying down of a growing bine and subsequent overlaying with soil. After the growing season the bine is unearthed, cut up and planted out."

Though Hornsey has modern science to back him up it's nothing new that hops are grown from cuttings. Burges (3) quotes T Mortimer writing back in 1761:

"Experiments have been made in raising a hop garden by sowing the seeds, but it turns to no account, because it is not only a tedious way, but the hops so produced will be of different kinds, and many of them wild and barren."

More detail is provided by Samuel Rutley (4) writing in 1848:

"I once grew a great many plants from the seeds of the Golding hop; there was nearly an equal number of male and female plants, but there was not one female plant that produced a hop at all like a Golding hop, nor was there a single plant amongst them all that produced a hop that I would have raised a plantation of, or was not very inferior to any hop I ever saw growing in a plantation."

It's interesting to see that in 1848 Rutley talks of male and female plants. The full significance of this was not fully known until 1900 with the re-discovery of Mendel's work on inherited traits and the understanding of genetics that came with it.

Each of the named 'varieties' of hops known to brewers are in fact cultivars, being genetically a single female plant cloned by taking cuttings. Well, not quite all of the 'varieties', as I've already mentioned there are ten different cultivars of Goldings in the National Hop Collection, but more on that later.

Despite the fact hops are commercially grown from cuttings it is still possible for genetic variation to occur through selection of mutants for propagation (7):

"Occasionally a plant will produce a bine which differs somewhat in character from the other bines; this is called a 'bud sport', or 'bud mutation'. The difference may be either in the direction of improvement or depreciation in the quality of the cones, or in the cultural character of the plant. If a cutting from this individual bine is used for propagation, the plant which is obtained, and its vegetative progeny, will retain it characters. In this way, by selecting bines with improved characters, new and better types of hops have been introduced from time to time."

The process of clonal selection is in fact necessary to maintain hop varieties (8):

"The production of bud sports and the intrusion of chance seedlings into gardens from which cuttings are taken for propogation leads, ultimately, to a mixture of types and, in the case of stray seedlings, to different varieties in what should be uniform stock. This has happened to most of the older varieties which are still in cultivation, and has made it necessary to select, from the mixed populations, plants having the desirable characteristics of the variety; than, after observation and testing over a number of years to raise clonal races from the best of them.

Hops can of course be grown from seeds, early varieties will have originated as landraces but are commercially grown from cuttings. Prior to the release of hops from the breeding programme by Wye College only two English varieties in commercial production were known to have been grown from seedlings: Fuggle and Whitbread Golding Variety.

Though my interest in the origin of hop varieties mainly relates to Goldings and the Farnham connection having got this far I'll add a few words on polyploidy for completeness' sake.

Normally hops are diploid, containing two sets of ten chromosomes in their cells, except in their sexual cells (pollen and eggs) which only contain one set and so are haploid. By chemically treating growing buds it is possible to produce cell with four sets of chromosomes in each cell (tetraploids). Why this is of interest to hop breeders is explanied by Burgess (9):

"The sexual cells of the 'tetraploid' form of a plant will, therefore, contain twenty chromosomes, i.e. the full number characteristic of the original diploid variety. If a tetraploid female hop is crossed with a diploid male, the cells of its progeny will contain three sets of chromosomes - two from the tetraploid female and one from the diploid male. They are, therefore, called 'triploids'. As they contain a full complement of chromosomes from the female plant, they will have a very much greater chance of resembling that parent. Dark proposed in this way, by using disease-resistant male plants, to produce disease-resisting varieties closely resembling existing varieties in their other general characters, but, because triploids are usually sterile, containing less seeds."

Burgess was writing when this research was relatively new but in the Barth-Haas Hops Companion published in 2009 there are triploid varieties such as Galaxy and Willamette listed so it seems it's proved a successful way of producing new varieties.

Right, I think I'm ready to get back to Goldings now...

References:

1. Hops, AH Burgess, p19.


2. Brewing, Ian S Hornsey, p60


3. Hops, AH Burgess, p44.


4. Journal of the Royal Agricultural Society of England, Volume 9, pages 545-546. Online here.


5. Journal of the Royal Agricultural Society of England, Volume 62, page 87. Online here.


6. Hops, AH Burgess, p42


7. ibid p38


8. ibid p38-39


9. ibid p50