seedlots and PSbMV-immune germplasm in northern India is a unique associa-
tion, probably not occurring in any other region in the world.
Notwithstanding the inadequacy of supplementary, definitive information, I
hypothesize that northern India indeed represents an original epicenter of
PSbMV from which the virus was disseminated to other parts of the world in
infected Pisum seeds.
Properties of present PSbMV strains suggest that an acquired viral
genetic ability to induce its transmission through host seeds would have
greatly favored survival of the virus. For instance, there are no known
non-Pisum, persistent inoculum-reservoir hosts of PSbMV from which disease
spread (by aphids) could occur, i.e., the virus perennates exclusively in
infected seeds. Without seedborne inoculum, therefore, survival of the
virus would have been dependent upon spread from a continuous supply of
infected annual host plants. The probability of occasional inoculum dis-
continuities, causing breaks in the disease cycle and loss of PSbMV, is
substantial. This deficiency would have been further aggravated bv a very
narrow host range, characteristic of present-day PSbMV strains. Survival
of PSbMV in its center of origin, then, would certainly have been favored
by seed-transmissibility in Pisum.
Development and proliferation of PSbMV-immune Pisum lines in northern
India (genotype sbm.sbm) (5) suggests that in some cases PSbMV immunity
conferred a yielding-ability advantage, compared to PSbMV-susceptible
lines. Such lines likely would have been noticeable to aboriginal growers
and could have been chosen for superior yields, without knowledge of cause.
At the same time, other Pisum lines providing continuous seedborne inoculum
may have tolerated infection and yielded satisfactorily despite the
presence of PSbMV. Both mechanisms proceeding simultaneously would thus
have accounted for the perpetuity of both PSbMV inoculum and PSbMV-immune
lines, to the time of Pisum germplasm collections in 1969-70.
Discussion. The antiquity of peas in northern India must be regarded
as relevant to the possible origin of PSbMV in that area. Centers of
earliest old-world occurrence of Pisum are still partially and only
generally defined (2, 6, 7). The general region of Afghanistan and India
is proposed as an "Eastern subcenter" of earliest occurrence, with Iraqi
Kurdistan, southern Turkey, and Ethiopia as a "Western subcenter". It is
therefore conceivable that Pisum could have occurred naturally in northern
India, in very early times. Ashraf (1) believes that if peas had been
introduced into northern India, such introduction may have been as long as
1,200 years ago, and is currently searching ancient Sanskrit manuscripts
for mention of pea-like plants. Peas could have been transported from
Turkey or Kurdistan through Afghanistan to India via established trade
routes in ancient times. However, knowledge of pea seeds in ancient trade
is either non-existent or very obscure. There appear to have been no
archeological investigations capable of indicating ancient food-use of peas
in India, certainly nothing equivalent to those (3) suggesting food-use of
peas in 7,000 BC in Iraqi Kurdistan.
