Morphological differences in pollen grains of Zea diploperennis, Tripsacum dactyloides, Tripsacum-diploperennis hybrids and maize
--M. Eubanks

The exine, i.e., outer layer, of the wall of a pollen grain in Zea mays L. and its wild relatives, teosinte and Tripsacum, has distinctive sculpturing of simple raised projections called spinules. Scanning electron microscopic characterization of spinule patterns and density has been used in palynological studies to identify modern and fossil maize pollen (Tsukada and Rowley 1964; Irwin and Barghoorn 1965; Banerjee and Barghoorn 1972; Grant 1972). In Zea, the spinules are more or less uniformly distributed over the surface of the pollen grain, whereas in Tripsacum, spinules are consistently clumped together and this forms a distinctive reticulate pattern.

Scanning electron microscopy was employed for investigation of pollen spinule patterns and density among Tripsacum dactyloides L., Zea diploperennis Iltis, Doebley and Guzman, Tripsacorn (T. dactyloides X Z. diploperennis), Sun Dance (Z. diploperennis X T. dactyloides), Zea mays, and an F2 maize diploperennis hybrid. Dry pollen grains were mounted, coated with gold-palladium alloy for conductivity, then photographed with high resolution SEM. Spinules were more or less evenly distributed in diploperennis, maize, and all of the hybrid plants, with the exception of Tripsacorn, which shows some clumping of spinules but not the degree of distinctive reticulation that occurs in Tripsacum. Spinule density was determined by counting the number of spinules in an area of 100 m2. See Table 1 for results.

Findings show Tripsacorn spinule density of 6.4/m2 is greater than the diploperennis male parent and less than the Tripsacum female parent as expected; thus spinule density is intermediate between both parents. Spinule density in Sun Dance is 4.0/m2. It is lower than the diploperennis female parent; significantly lower than the Tripsacum male parent, and lower than Tripsacorn, the reciprocal hybrid between the same two parents. Pollen spinule density is evidently a characteristic that is distinctive between the different hybrids Tripsacorn and Sun Dance. Maize spinule density is 5.7/m2. The maize-diploperennis hybrid at 5.3/m2 is intermediate between both its parents. Additional SEM study is being carried out to investigate potential significance of these findings for pollen identification.

Table 1. Spinule density in Zea, Tripsacum and Tripsacum-Zea hybrids.

Type                                 No. spinules/100 m2            No. spinules/m2
Tripsacum dacyloides             1,144                                   11.2
Zea diploperennis                     466                                      4.6
Tripsacorn                                    650                                      6.4
Sun Dance                                    409                                      4.0
Zea mays                                  584                                       5.7
Maize X diploperennis              541                                       5.3

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