BIOSYSTEMATIC STUDIES IN THE BOUTELOUA CURTIPENDULA COMPLEX. III. POLLEN SIZE AS RELATED TO CHROMOSOME NUMBERS

Pollen size statistics are presented for 10 closely related species of Bouteloua and relationships between pollen size and chromosome numbers are presented for 13 populations of 5 species and 3 varieties. With 1 exception, all populations of all taxa conformed to a general pattern of pollen size dependent upon chromosome number. Chromosome numbers varied from 2n = 20 to 2n = ca. 103, with several independent aneuploid series. Statistical analyses were made of pollen size as related to chromosome number in the 3 varieties of B. curtipendula. These data showed that tetraploids (2n = 40) of var. tenuis had significantly greater pollen size and coefficient of variation than diploids (2n = 20) of the same variety. Similarly, aneuploids of var. curtipendula with 2n = 45 to 2n = 64 chromosomes had significantly larger and more variable pollen than tetraploids (2n = 40) of the same variety. Highly significant positive regression coefficients were obtained from analyses of chromosome numbers and mean pollen size, and chromosome numbers and coefficient of variation, for var. curtipendula. Regression coefficients for var. caespitosa populations with chromosome numbers over the hexaploid (2n = 60) level were not significant. THE TAXONOMIC and probable phylogenetic relationships of the polymorphic grass Bouteloua curtipendula and 11 closely related species are being published concurrently (Gould and Kapadia, 1964). Three varieties were recognized in B. curtipendula, these based mainly on morphological and cytological characteristi'cs. The typical variety, var. curtipendula, includes tetraploids (2n = 40) alnd aneuploids with chromosome numbers from 2n = 41 to 2n = 64. A second variety, var. tenuis, is comprised of diploids (2n = 20), tetraploids, and aneuploids with 2n = 41 or 2n = 42. The third variety, var. caespitosa, for the most part is made up of plants with high aneuploid chromosome numbers (2n = 69 to ca. 103) and unequal segregationi of chromosomes during PMC2 division I. A few Mexican plants are included in this variety that have 2n58-64 chromosomes and essentially equal segregation of chromosomes during PMC division 1. Diploid or near-diploid chromosome counts are reported for 4 of the 11 species assumed to be closely related to B. curtipendula and a fifth species was determined to be tetraploid. The belief is expressed that probably all 6 of the remaining species also are diploids. This paper presents pollen size data and a statistical evaluation of the relationship of chromosome n-umber and pollen size in the B. 1 Received for publication June 29, 1963. Technical Article No. 4489 of the Texas Agricultural Experiment Station, College Station, Texas. Research supported by grants G-6372 and G-19428 from the National Science Foundation. Based in part on a dissertation submitted to the Graduate School of Texas A & M University in partial fulfillment of the requirements for the degree of Doctor of Philosophy. 2 PMC = pollen mother cell. curtipendula complex. An effort has been made to determine the reliability of pollen size as a taxonomic character and as an indication of ploidy level. Pollen size has been shown to be correlated with chromosome number and ploidy level in a number of genera of grasses. Nissen (1950) obtained a highly significant positive correlation (r = 0.79, P < 0.001) for an aneuploid series of Poa pratensis L. from southeastern Norway. Positive correlation between pollen diameter and chromosome number in Poa pratensis also was found by Akerberg (1942). L6ve (1952) obtained similar results with Poa irrigata sensu L6ve. In earlier studies of Bouteloua curtipendutla, Gould (1959) mapped the distribution of presumably apomictic plants with high aneuploid chromosome numbers on the basis of the size and variation pattern of pollen grains. Working with a series of related Andropogon taxa, Gould (1953, 1957) found striking differences in pollen size between plants of 3 levels of ploidy, and used pollen size to separate morphologically similar taxa with different chromosome numbers. More recently (1963), the same author reported differences in pollen size between plants of 2 closely related species of Digitaria, one of which, D. sanguinalis (L.) Scop., is tetraploid, with 2n = 36 chromosomes, and the other, D. adscendens (H.B.K.) Henrard, is hexaploid with 2n = 54 chromosomes. Bell (1959), working with tomato, dill, and Portulaca plants, found considerable variation in pollen size resulting from different levels of mineral nutrition. Such extensive nutritional variations, however, would not be encountered by many plants in their natural habitats. The study by