Perennial teosinte-Gaspe hybrids: inheritance of pollen grain size

In the Maydeae (Gramineae), maize (Zea mays) is the species which possesses pollen grains of great size. Within the same tribe, the species pertaining to the Tripsacum genus are the ones which have the smallest pollen grains, while the teosintes (Z. mexicana, Z. perennis and Z. diploperennis) have intermediate pollen grains between those of maize and Tripsacum. The variety of size of pollen grains among species is closely related to the length variation of the silks. Our well-known maize, with long silks, possesses large pollen grains, with sufficient reserve to produce long pollen tubes. Contrary to this, the Tripsacum species, which have short silks, have small pollen grains, with a capacity to produce only short pollen tubes.

Teosintes, species closely related to maize, develop silks intermediate in length between those of maize and those of Tripsacum, and similarly their pollen grains are also intermediate to those species. From what has been explained, it may be considered that the size of pollen grains is a specific character which is closely related to the reproductive needs of the species. The object of this report is to present results obtained in the study of the inheritance of this specific character (size of pollen grain) in hybrids between perennial teosinte (Z. Perennis) and Gaspe (Z. mays).

The experimental results (size of pollen grain) were obtained on the basis of the individual plant, and originate from the analysis of that character in 10 perennial teosinte plants, 11 Gaspe plants, 110 F1 plants and 605 F2 plants produced by crossing perennial teosinte and Gaspe. The four populations studied were cultivated in the corresponding cycle during 1982/1983. The pollen was harvested on each tassel at the moment of natural dehiscence. Each pollen sample was treated with a mixture of sol. I2-KI and lactic acid in a ratio 1:1 v/v. Measurements were made on 20 pollen grains of each plant, taken at random, by means of a microscope provided with a micrometric ocular, and a magnification of 400 X. The size of the pollen grain was evaluated upon the biggest diameter. Measurements of the pollen grain size based only on the largest diameter are approximately 8% higher with respect to the average of the largest and smallest diameter.

Perennial teosinte has significantly smaller pollen than Gaspe (see Tables 1 and 3). The average size of the pollen grain for the F1 population is more similar to perennial teosinte than to Gaspe. The average size of the pollen grain for the F2 population approaches the theoretical average among its progenitors (see Tables 1 and 3). From the results obtained in Table 2, heritability was calculated, this being high (0.82).

The frequency distribution of pollen grain size in the four populations studied is presented in Figure 1. The parents are separated in size, and the F1 is more variable than perennial teosinte but with a distribution and mean nearer to this species. The F2 presents an ample distribution which exceeds parental extremes.

The data obtained indicate that: (1) the size of pollen grain is inherited quantitatively; (2) the size of F1 pollen grains is closer to teosinte, probably due to the different chromosome contribution of both species (20 teosinte chromosomes, 10 maize chromosomes) and consequently the character is controlled by the mother's genotype; (3) the size of the pollen grains of the F2 does not differ significantly from the theoretical mean, showing that genes with additive effect prevail. The frequency distribution of the F2 is transgressive in both ways, that is to say, there are plants with smaller pollen than perennial teosinte and larger than Gaspe.

Table 1: Number of plants (N) and mean pollen grain size (PGZ) in perennial teosinte (Zp), Gaspe (Gs) mid its F1 and F2 populations.

Table 2: Means and variances of perennial teosinte (Zp), Gaspe (Gs) and its F1 and F2 populations.

Table 3: Differences between means for PGZ of perennial teosinte (P1), Gaspe (P2) and its F1 and F2 populations.

Figure 1.

Jorge Luis Magoja and Ida Graciela Palacios
 
 


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