Continued study of a defective WF9 cytoplasm, "wsp"

A cytoplasmically inherited syndrome, called "wsp", traces back to a single open pollinated plant of WF9 (Duvick, 1958 and 1961 Coop Newsletters). The stock has been maintained by selfing; via backcrossing its nuclear genotype also has been placed in "normal" WF9 cytoplasm.

Starting with the wsp cytoplasm line as original female, the inbred SK2 was crossed and backcrossed repeatedly (through 12 backcrosses) as pollen parent, giving a line - SK2wsp - with SK2 nuclear genotype and wsp cytoplasm. The phenotype of SK2wsp is identical to that of SK2; it shows no wsp traits. In fact, during all backcross generations SK2wsp plants have never shown any wsp traits. (Typical "wsp" plants have reduced vigor and a distinctive pale green streaking of the leaves; hence the name "wsp", signifying "weak streaked plant".)

SK2wsp and SK2 were each crossed as female to a normal appearing strain of WF9 and the two F1s were (1) selfed, (2) backcrossed as female to WF9, and (3) backcrossed as female to SK2. Both F1s were of normal (non-wsp) phenotype. The resulting F2 and backcross progenies were grown out at Johnston, Iowa in 1985 and plants were classified for presence or absence of the wsp phenotype.

Results are summarized in Table 1. They show that the wsp phenotype appeared in WF9 backcrosses and F2 progenies when they were in wsp cytoplasm, but not when they were in SK2 cytoplasm (A possible exception is the appearance of 3 putative wsp plants in one F2 progeny with SK2 cytoplasm.) The wsp phenotype was not seen in any SK2 backcross progenies, in either wsp or SK2 cytoplasm.

These data indicate that nuclear genotypes modify the expression of wsp, more or less as nuclear genes modify the expression of cytoplasmic male sterility. The nuclear genotype of SK2 dominantly suppresses expression of the wsp phenotype; it acts as a "restorer" line. The nuclear genotype of WF9 facilitates expression of the wsp phenotype but is recessive to SK2's nuclear suppressor genotypes; it acts as a "maintainer" line. However, precise genetic mechanisms are not clear in these data. Ear-rows varied widely in expression of wsp, for example, even though they should have had essentially identical nuclear gene segregation patterns.

Also, as in cytoplasmic male sterility, the data indicate that wsp cytoplasm seems to maintain its genetic identity during many generations of nuclear suppression of expression of the wsp phenotype. Its presence can be revealed by substitution of a "maintainer" genotype such as that of WF9.

The WF9 and SK2 plants used for backcrossing were selfed and their progenies were grown out alongside the backcross and F2 progenies. One WF9 family unexpectedly produced a large number of plants with the characteristic wsp phenotype. All but one of these wsp plants were in 2 of 5 ear-rows that arose from a single ear-row (of normal appearance) in the previous generation.

Backcross progenies made with pollen of the 2 "wsp-progeny" WF9 plants had wsp phenotypes only when in wsp cytoplasm, never in SK2 cytoplasm. They did not differ in this respect from backcross progenies made with pollen of "normal-progeny" WF9 plants; these backcrosses also had wsp phenotypes only when in wsp cytoplasm, never in SK2 cytoplasm. It appears, therefore, that the wsp phenotype of the aberrant ear-rows is not transmitted through the nucleus, at least not as a genetic dominant. Perhaps WF9 has again given rise to a wsp cytoplasm.

Open pollinated ears have been saved from all wsp plants vigorous enough to make an ear (many plants were barren). Seed from these ears will be planted out in 1986 for observation and controlled pollination. Tests will be set up to determine whether or not their wsp syndrome is inherited, if it is cytoplasmic, and if so whether or not it differs from the original wsp cytoplasm.

Table 1. Classification of progenies for presence or absence of wsp phenotype.

Donald N. Duvick
 
 


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