J. Genet. 24: 117-301 (1931)
The Genetics and Cytology of Dahlia Variabilis
W. J. C. Lawrence

(John Innes Horticultural Institution, Merton)

(d) Mosaicism.

In mosaic forms the distribution of the anthocyanin is discontinuous in both stems and ray florets. It occurs in flecks and streaks and sometimes in larger areas (Plate IX, figs. 12 and 13). The flavone ground of the rays is never mosaic. The size of the coloured area may vary from one cell to the whole of the capitulum. Both the pale and deep pigments may be mosaic, but the pale anthocyanin is never mosaic unless mosaic deep anthocyanin is present. This is demonstrated particularly in families segregating forms with pale anthocyanin only, mosaic forms with deep anthocyanin only, and mosaic forms with both pale and deep. In this last class both pigments are indiscriminately mosaic; in the mosaic individuals with deep anthocyanin only, absence of the deep pigment reveals the clear yellow or ivory ground colour; but all the pale individuals are self-coloured, and crossed between themselves and with normal-coloured forms never give mosaics.

Two mosaic seedlings have been extensively used, namely M5 and M8. The colour of M5 is crimson-scarlet and in the absence of this the ground is yellow, i.e. there is no trace of the pale anthocyanin. In 1926 M5 was crossed with another mosaic, M1 (purple on a magenta ground). From this cross (27/27) a number of mosaic and self-coloured individuals were raised. The self-coloured plants were of two kinds: (a) purplish or (b) apricots of various shades. Three of the purplish and four of the apricots were selfed and crossed with various seedlings known to be normal. 167 plants were raised from the apricots and all of those with anthocyanin colouring were pale and normal. The three self-coloured purplish seedlings were similarly crossed and 57 plants raised, all of which were normal.

It is clear therefore that (1) self-coloured derivatives of mosaic forms breed true for normal self-colour, and (2) pale-coloured forms do not give deep-coloured ones.

*In any appropriate family mosaic sectors are occasionally found, but these are clearly analogous with the type of sectorial mutation discussed later.

M5 was further tested by crossing to a normal yellow seedling 14/26 (Y3yIi). Twenty-two mosaic and 35 normal plants were raised (family 15/28), the normal plants comprising 33 yellow and 2 crimson individuals. Two ground colours and two corresponding anthocyanin colours were found in this family, one ground colour being a little paler than the other. On the paler yellow the anthocyanin appeared crimson; on the deeper yellow it was crimson-scarlet. No pale anthocyanin occurred. One of the self-coloured crimson seedlings (15/28) was back-crossed with its yellow female parent 14/26. The progeny (family 3/29) consisted of yellows and scarlets, and all the forms with anthocyanin were moderately deep coloured and normal, no mosaicism occurring*.

This F2 may therefore be taken to substantiate the true-breeding nature of self-coloured derivatives of mosaic forms. These two families 15/28 and 3/29 also disclose another fact—that M5 is recessive for the A anthocyanin factor.

Including 15/28, six forms were raised in which M5 was crossed to 14/26 (yellow) or White Star (ivory). No individuals with pale anthocyanin were found in a total of 263 plants. Moreover the ratio of individuals with anthocyanin to those without is 130:133. This must mean that M5 is at least simplex for B, and its constitution for flower colour may be written provisionally as YYyyI-aaaaBbbb. Only two anthocyanin colours are found in these six families—crimson-scarlet and crimson. Empirical evidence strongly suggests that the factor B is cumulative in effect, and if this is so it is highly improbable that M5 is duplex for B, otherwise deeper or paler colours than crimson and crimson-scarlet would be found in the progeny of M5.

This tentative hypothesis has an important bearing on the nature of the inheritance of mosaicism in Dahlia, for M5 gives a very close approximation to expectation on the assumption that it is simplex for B, therefore the inheritance of mosaicism in Dahlia may be Mendelian and entirely free from the effects of somatic segregation.

Before turning to the inheritance of mosaicism, another mosaic seedling M8 (purple on pale magenta) must be mentioned. Crossed with the normal plants 14/26 (yellow) and White Star (ivory) it gave eight seedlings without anthocyanin in a total of 241. Four of these individuals were from crosses in which the female parent was without anthocyanin, and four from crosses in which the female parent had anthocyanin. Therefore all could not have arisen from accidental self-pollination. It is probable that these eight seedlings were an extreme condition of mosaicism, for among the progeny of M8 twelve individuals were noted with only one or two solitary spots of anthocyanin, and in many others the coloured areas were very few. If this is so, then M8 is triplex or quadriplex for A. Of the plants with authocyanin 186 had the deep and 55 the pale pigment. Again it is possible that the pale individuals are extreme examples of mosaicism. Hence the significance of the ratio of 186 BA to 55 bA plants cannot at present be decided, but it is more probable that it indicates that M8 is duplex for B, than simplex.

I have already stated that the deep pigment suffers considerable bleaching as the end of the growing season approaches. In two families, 28/27 and 33/27, both crosses between apparently normal plants, mosaics appeared. 28/27 gave 16 self-coloured, 6 mosaics, and 4 individuals which were scored as self-coloured early in the season, but later showed mosaic beneath the somewhat bleached deep pigmentation. Similarly in 33/27 the progeny consisted of 30 self-coloured, 1 mosaic, and 5 individuals showing mosaic at the end of the season. It is possible that these latent mosaics are due to the differential action of the mosaic factor upon the A and B pigments, but further work is necessary to establish this.

Family No. of plants Parents No anthocyanin Self-coloured deep anthocyanin Mosaic anthocyanin Variation of percentage anthocyanin in mosaic seedlings Average percentage anthocyanin
46/30 53 14/26 (normal) x M5 (8-10% anthoc.) 29 0 24 10-75 13
7/29 6 M5 (12% anthoc.) x 14/26 (normal) 2 0 4 15-25 15
48/30 65 14/26 (normal) x M5 (50% anthoc.) 31 3 31 1-70 15
47/30 65 14/26 (normal) x M5 (self-coloured)
   =100% anthoc.
28 6 31 10-70 22
45/30 17 White Star (normal) x M5 (self-coloured) 10 2 5 10-45 19
12/29 43 36/26 (normal) x M5 (self-coloured) 2 24 17 Trace-70 15
52/30 60 14/26 (normal) x M8 (12-15 % anthoc.) 0 8 46 Trace-90 31
51/30 41 14/26 (normal) x M8 (30% anthoc.) 0 3 31 Trace-90 29
50/30 52 14/26 (normal) x M8 (self-coloured) 3 17 32 Trace-90 28
49/30 50 White Star (normal) x M8 (self-coloured) 1 4 29 2-75 20
9/29 12 MS (mosaic) x Self 0 7 5 10-90 73

The inheritance of the mosaic character is shown in Table XI. The degree of mosaicism was determined by careful inspection of the whole of each plant at full bloom. The extent of the coloured area (referring to anthocyanin pigmentation only) is given as a percentage of the total area of the ray florets. In the case of M5, capitula showing from approximately 10 to 100 per cent. anthocyanin pigmentation in the rays were used, and the total percentage of anthocyanin in the families ranged from 13-22. The total percentage seems to increase slightly with the percentage pigmentation in the parent capitulum.