value in a multifactorial system than in the 
monofactorial system hitherto used so widely 
in Europe. 
SELECTION PRESSURES IN 
RACE FORMATION 
It may be significant that physiologic races 
of nematodes have been found primarily in the 
genus Heterodera (21, 25, 26, 34, 35, 36, 51, 
73, 74, 78, 79, 116, 132) and the genus 
Meloidogy ne (31, 32, 49, 85, 88, 110, LOS 138, 
148), The exceptions include evidence of phy- 
siologic races in Ditylenchus species (12, 76, 
125) and in Radopholus similis (Cobb, 1893) 
Thorne, 1949, the burrowing nematode (33). 
In both Meloidogyne species and Heterodera 
species the most critical selection pressure 
that provides the direction for genetic change 
has probably been enforced starvation. In 
Heterodera, because the host range in very 
narrow, the growing of resistant hosts has 
the effect of cutting off the food supply. 
Nematodes are obligate parasites (113); under 
conditions of such drastic environmental 
changes, populations will either become re- 
duced in size to the point of extermination or 
the species will evolve in response to this 
change. Any surviving individuals must, of 
necessity, possess preadaptive morphological 
or physiological characters in the direction of 
the environmental change. 
In Meloidogyne, the incidence of physiologic 
races has been noted primarily under conditions 
where their natural host range was forcibly 
restricted by greenhouse culture or by culture 
in restricted field plots. Thus, although Giles 
and Hutton (49) could demonstrate a breakdown 
in a resistant tomato variety under restricted 
plot culture in 5 years, they found no such 
breakdown in the same tomato variety when it 
was repeatedly grown under commercial field 
conditions with a susceptible tomato variety. 
Similar results were found by Allard with lima 
beans (3). The formation of physiologic races 
could therefore be the result of a restricted 
host range, either natural or artificial. Awide 
host range is apparently not conducive to race 
formation, because selection pressure exerted 
by starvation is weak or absent. 
In theory it should be possible to prevent or 
retard race formation in nematodes by reducing 
129 
the selection pressure in that direction. The 
results of Giles and Hutton with resistant 
tomatoes, and those of Allard with lima beans, 
indicate that when these two crops are grown 
in the field, physiologic races present no 
problem. Can the explanation be that, under 
field conditions, the nutritional needs of root- 
knot nematodes are met by other equally 
acceptable plants such as weeds? Likewise, 
with the golden nematode, would a rotation, 
alternating resistant with susceptible varieties, 
slow down the formation of resistance-breaking 
races? There is need for additional experi- 
mental evidence here, although limited experi- 
ments from the Netherlands, England, and 
elsewhere would indicate great promise that 
the answers will soon be forthcoming. In this 
country resistance-breaking races have been 
shown to compete successfully with the parental 
populations (148), 
It is possible that some of our chemical 
control measures have actually removed 
natural selection pressures. Whitehead (145) 
has suggested that soil fumigation kills soil 
fauna indiscriminately, As a result, the slow- 
growing nematode predators are killed, and 
fast-growing parasitic nematodes survive to 
dominate (145). 
GENETIC RESEARCH WITH 
NEMATODES 
Despite the importance of genetic variability 
in nematodes, genetic studies of nematode 
populations are almost totally unknown today. 
In a careful search of the literature few in- 
stances of research in interspecific hybridiza- 
tion were found. Sperm cells of the sugarbeet 
nematode, Heterodera schachtii Schmidt 1871, 
were found in the odcytes of the clover cyst 
nematode, Heterodera trifolii Goffart 1932 
(94, 95, 96). Second-generation females pro- 
duced four types of larvae: Small, normal, 
giant, and morphologically abnormal. It was 
suggested that these changed populations arose 
from either gene mutation or polyploidy. 
Cytological studies in the family 
Heteroderidae have demonstrated the occur- 
rence of both parthenogenesis and sexual 
reproduction in this group of nematodes (24, 
111, 135, 136, 137). In addition, various de- 
grees of ploidy were discovered (136). 
Although the appearance of physiologic races 
in Heterodera rostochiensis was noted nearly 
