Many researchers have observed variable clonal striping on leaves within lines of Mu plants (D.S. Robertson, MNL 55:2, 1981; M. Freeling, Ann. Rev. Plant Phys. 35:277, 1984). These variable clonal stripes are a type of dominant somatic leaf striping seen in active Mu plants. Most of the stripes are small, although occasionally a white or yellow clonal stripe may be wide enough to cover from 1/8 to 1/4 of a single leaf. In general, inactive derivatives of Mu lines do not exhibit such clonal striping (D.S. Robertson, MNL 57:4, 1983; V. Sundaresan, unpublished data). If the DNAs from the striping sectors and from the normal green tissue from the same leaf are examined by Southern blot analysis, differences in the pattern of Mu-hybridizing restriction fragments are observed (M. Alleman, Ph.D. Thesis, U. of California, Berkeley, 1984). These observations suggest that the clonal stripes result from the activity of the Mu elements. Robertson (op. cit.) has observed that there is considerable variability in the striping exhibited within the same Mu lines when grown at Berkeley or at Ames, with the striping observed at Berkeley being significantly greater. He has suggested that this difference might result from the lower temperatures in Berkeley. Further investigation showed that lower temperatures did in fact cause increased striping in Mu plants, (D.S. Robertson, MNL 56:2, 1982); however, this appeared to be true for some normal plants as well, and therefore it was not clear that Mu activity was influenced by low temperature. In this investigation, we re-examine the effect of temperature on Mu activity. In the experiment, three identical growth chambers were set up at three different temperatures and plants were grown in the chambers at mean temperatures (day + night/2) of 19 C, 24 C, 29 C. The warmest chamber was actually started at 34 C for two days after planting, and then turned down in order to avoid heat damage to the seedlings. There were a total of twelve maize lines used, including 282 plants that were distributed among the three temperatures; this included active Mu lines from Freeling (Mu, 1s2p) and Robertson (Pl Mu) stocks, and inactive derivatives of these lines that served as controls. All of the active lines displayed a high degree of clonal striping at the coldest and warmest temperatures but striping at the intermediate temperature was infrequent. By contrast the inactive Mu lines exhibited little or no striping at any temperature. The actual breakdown of the number of plants that had clonal striping at each of the temperatures is shown in Table 1.

Table 1. Ratio of striped plants to total plants.
 

Line	Activity*	Pedigree	19 C	24 C	29 C
A	+	Pl Mu	2/5	015	2/5
B	-	Mu, 1s2p	0/13	0/13	2/39
C	+	Mu, 1s2p	6/8	2/9	3/5
D	+	Mu, 1s2p	4/15	1/16	5/14
E	+	Mu, 1s2p	1/5	0/6	0/3
F	-	Pl Mu	1/3	1/5	0/6
G	-	Pl Mu	0/4	0/4	0/4
H	-	Mu, 1s2p	0/4	0/4	0/5
I	+	Pl Mu	2/4	0/4	0/4
J	+	Mu, 1s2p	3/6	0/4	0/6
K	+	Pl Mu	1/5	0/6	2/6
Total for Active Mu lines			19/48	3/50	12/43
Total for Inactive Mu lines			1/24	1/26	2/54

*+ = active Mu
- = inactive Mu

From the data, which include all plants tested in 1987, there was only one obvious trend, which was the consistent appearance of clonal stripes in active Mu plants in the warmest and coldest chambers. These results, though not complete, suggest that Mutator activity is greater at temperature extremes. The results obtained in this study could be viewed in the light of McClintock's hypothesis of "genome shock". McClintock (Science 226:792, 1983) has suggested that the activity of transposable elements could be regulated by stresses to the organism and that such stresses could serve as a mechanism for the reorganization of the genome. An increase in Mutator activity when plants are grown under sub-optimal conditions would be consistent with this hypothesis.

Kevin Nelson Heller
 
 

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