Agriculture Reference
In-Depth Information
to choose material suited to the purpose of the survey. In the annual study of barley
powdery mildew undertaken by the UK Cereal Pathogen Virulence Survey
(UKCPVS), for example, the differential set includes not only cultivars with known
resistance genes but also two breeding lines and one landrace which express certain
resistance genes clearly, as well as several cultivars with unknown resistances and
others which are of interest because they are resistant to the current pathogen
population (Slater, 2005b).
(b) Resistance genes in differential varieties
If a differential variety is known to have one particular resistance gene, it may then
be postulated that the pathogen has the corresponding avirulence or virulence gene.
It is often assumed that near-isogenic lines are most suitable for identifying
pathogen virulences because each line should have a single specific resistance gene.
In fact, this is not always so. Some of the Pallas near-isogenic lines of barley, for
instance, were known to have more than one mildew resistance gene when they were
released (Kølster
et al.
, 1986) and other specificities have since been discovered.
The Pallas line P17, for example, was thought to have only
Mlk1
(formerly
Mlk
) but
was later found to have an additional specificity, called
Ml(P17)
(Brown
et al.
,
1996). Some isolates which are virulent on
Mlk1
and on the UKCPVS differential
Hordeum 1063 are therefore avirulent on P17.
Inevitably, near-isogenic sets cannot include all recently discovered resistance
genes. Many modern, European spring barley varieties have
Ml(Ab)
mildew
resistance, first used in the variety Triumph (Brown and Jørgensen, 1991; Slater,
2005b). As there is no Pallas near-isogenic line with
Ml(Ab)
, barley mildew workers
may include either Triumph, which has
Mla7
as well as
Ml(Ab)
, or Lotta (formerly
Sv.83380), with
Ml(Ab)
only, in their differential sets (e.g. Wolfe
et al.
, 1992).
Nevertheless, it is fair to say that there is usually greater uncertainty about the
resistance genes in a differential set of varieties than in a near-isogenic set,
especially when varieties are included in the set because they show differential
interactions with some pathogen isolates but little or nothing is known about the
genetics of their resistance. One of the best characterised differential sets is that used
in the UK barley mildew survey; not only are the resistance genes in the core set of
varieties known - in most cases, they can be traced back to the introduction of the
genes into plant breeding programmes - but most lines have only one gene (Slater,
2005b). In the UK wheat mildew differential set, by contrast, several varieties have
poorly characterised resistances (Slater, 2005a). Attempts to resolve ambiguities in
some sets have been made. For example, the international differential set of wheat
varieties for resistance to yellow (stripe) rust (caused by
Puccinia striiformis
f.sp.
tritici
) includes Heines VII, which has the resistance gene
Yr2
. Subsequently, Heines
VII was found to have another resistance specificity, which is effective against
isolate WYR 85-22 from Ecuador but does not differentiate European isolates.
Isolates may therefore be avirulent on Heines VII even if they are virulent towards
Yr2
(Johnson, 1992; Calonnec
et al.
, 1997a).
