## Nomenclature

The best nomenclature for vertical resistances and parasitic abilities is one that names the genes. The two matching genes of a pair are given the same name, which immediately reveals that the two genes match each other. A vertical pathodeme or vertical pathotype can then be named according to the vertical genes that it is carrying, and it is immediately clear whether a pathotype does or does not match a pathodeme.

The Black et al (1953) nomenclature labels pairs of matching vertical genes numerically, in the order of their discovery, using an arithmetic series of numbers (i.e., 1, 2, 3, 4, etc.). The Habgood (1970) nomenclature uses the numbers of the binomial expansion (i.e., 20, 21, 22, 23, etc., with arithmetic values of 1, 2, 4, 8, etc.). Each binomial number has an arithmetic value that is double that of its predecessor. An advantage of the Habgood nomenclature is that the sum of any combination of binomial numbers is unique. For example, the sum 21 can be obtained only by adding 16 + 4 + 1, and no other combination of binomial numbers can add up to this sum. Habgood recognised the value of this uniqueness, which is essential to any system of naming.

Habgood originally applied his nomenclature to the host and parasite differentials, but Robinson (1976) applied it to matching pairs of vertical genes. Each pair of matching genes is then labelled with the binomial numbers 1, 2, 4, 8, etc., in order of discovery. The name of each pair of genes is the primary Habgood name, and it is a single binomial number. Any combination of genes is named with the sum of their several binomial numbers (Fig. 4.3), and this is a secondary Habgood name. It will be seen that any combination of genes, in either the host or the parasite, is named with a single number, and that exactly matching vertical resistances and vertical parasitic abilities have the same name (Fig. 4.4).

OLD |
NEW | |||

Host |
Parasite |
BLACK |
HABGOOD | |

name & genes |
genes |
name | ||

P |
IV |
0 | ||

a |
1 |
1 |
1 | |

2 |
2 |
2 | ||

1 2 |
1 2 |
3 | ||

3 |
4 |
4 | ||

1 3 |
1 4 |
5 | ||

2 3 |
2 4 |
6 | ||

12 3 |
12 4 |
7 | ||

Y |
4 |
8 |
8 | |

I |
XIX |
1 4 |
1 8 |
9 |

2 4 |
2 8 |
10 | ||

12 4 |
12 8 |
11 | ||

3 4 |
4 8 |
12 | ||

1 3 4 |
1 4 8 |
13 | ||

2 3 4 |
2 4 8 |
14 | ||

12 3 4 |
12 4 8 |
15 | ||

E |
il (I)* |
5 |
16 |
16 |

C |
ill |
1 5 |
1 16 |
17 |

D |
i (II)* |
2 5 |
2 16 |
18 |

L |
XVII |
12 5 |
12 16 |
19 |

G |
VII (III)* |
3 5 |
4 16 |
20 |

13 5 |
1 4 16 |
21 | ||

H |
VIII (IV)" |
2 3 5 |
2 4 16 |
22 |

V, Z |
XII |
12 3 5 |
12 4 16 |
23 |

J |
XV |
4 5 |
8 16 |
24 |

W |
X |
1 4 5 |
1 8 16 |
25 |

y |
XXIV |
2 4 5 |
2 8 16 |
26 |

o |
XXIII |
12 4 5 |
12 8 16 |
27 |

3 4 5 |
4 8 16 |
28 | ||

1 3 4 5 |
1 4 8 16 |
29 | ||

T,X |
XIV |
2 3 4 5 |
2 4 8 16 |
30 |

s,u |
XVI |
1 2 3 4 5 |
12 4 8 16 |
31 |

- Mayne's (1936) designations
- Mayne's (1936) designations

Figure 4.3 Nomenclature in the gene-for-gene relationship of coffee leaf rust (Hemileia vastatrix)

In the original nomenclature, host differentials were labelled with letters of the alphabet, and rust differentials were labelled with Roman numerals (red in the diagram). With this system, it was extremely difficult to determine how many genes, and which genes, were present in each differential. (Continued)

In the system used by Black et al (1953), each gene (green in the diagram) is labelled with a numeral in an arithmetic series (i.e., 1, 2, 3, 4, etc.). Matching genes are given the same number, and it is immediately apparent whether or not a particular vertical pathotype matches a particular vertical pathodeme, regardless of how many genes may be present.

In the (modified) Habgood (1970) nomenclature, the genes (blue in the diagram) are labelled with the numbers of the binomial expansion (i.e., 20, 21, 22, 23, etc.) which have arithmetic values of 1, 2, 4, 8, etc., each value being double that of its predecessor. These single gene names are the primary Habgood names. Secondary Habgood names label combinations of genes, and they are the sum of the arithmetic values of all the genes present. Thus

Habgood 21 labels the combination of genes 1, 4, and 16. The advantage of this system is that the sum of any combination of gene values is unique. However, the main value of the Habgood nomenclature is that it led to the discovery of the Person/Habgood differential interaction (see Fig. 4.4).

The composition of a secondary Habgood name is easily determined. Suppose the secondary name was 29. The largest possible binomial number is subtracted from it. In this case, this would be binomial 16. This means that gene 16 is present. The remainder is 13, from which 8 can be subtracted, indicating that gene 8 is present. The remainder is now 5, showing that genes 4 and 1 are also present. These gene names 16 + 8 + 4 + 1 add up to 29, and no other combination of binomial numbers can add up to this sum.

The Habgood nomenclature is useful for small numbers of pairs of genes. However, in any gene-for-gene relationship that has more than a few pairs of genes, the system becomes cumbersome because the Habgood numbers are so large. In wheat stem rust (Puccinia graminis tritici) for example, there are some thirty pairs of genes resulting from inter-specific hybridisation. This means that there are 230 (i.e., 1,073,741,824) possible gene combinations and the binomial numbers become impracticably large. There is then no useful alternative to the Black nomenclature.

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