POLLEN COMPETITION AND RELATEDNESS IN HAPLOPAPPUS SECTION ISOPAPPUS (COMPOSITAE). II.

Pollen competition in mixed pollinations involving a time handicap was studied among the five aneuploid members of section Isopappus of Haplopappus. Foreign pollen was applied to the stigmas of a plant followed after a delay of 0-180 min by domestic pollen. Domestic pollen was prepotent over foreign pollen in all cases, but with increasing delay an increasing percentage of hybrids appeared in the progeny. The time interval required to produce 50% of hybrids in the progeny (at which time the two pollen types were competitively matched in potency) was determined for each cross. The interval is correlated with relatedness, with chromosome number, and with length of the style of the pollen parent. It can be used as an estimate of the potential frequency of hybridization in nature, with all other factors being equal. SECTION ISOPAPPUS of Haplopappus (Compositae) consists of five taxa which are characterized by an aneuploid chromosome series of n = 7, 6, 5, and 4 (two of the taxa are n = 5). Three of the five are very similar morphologically and have been treated by the author (Smith, 1965) as subspecies of Haplopappus validus (Rydb.) Cory: subsp. graniticus E. B. Smith, n = 7; subsp. torreyi E. B. Smith, n = 6; subsp. validus, n = 5. The fourth, H. divaricatus (Nutt.) Gray with n = 4, is similar to H. validus, but distinct enough to deserve separate status at the species level. The fifth taxon, H. rigidifolius E. B. Smith with n = 5, is rather distinct morphologically from the other four. Thus, there are basically three levels of morphological distinctness in the section as reflected in the nomenclature of the taxa: (1) close (the subspecies of H. validus), (2) intermediate (H. divaricatus/H. validus), and (3) relatively distant (the validus-divaricatus taxa/ H. rigidifolius). The genetic relatedness of the taxa in the section generally parallels morphological distinctness (Smith, 1965, 1966). About three years ago, I became interested in determining whether or not the relative competitive ability of pollen in mixed pollinations involving a time handicap would parallel the morphological and genetic relatedness determined previously by more conventional taxonomic approaches. In a preliminary paper on this topic (Smith, 1968a), early data indicated that pollen competitive ability did, indeed, show such a parallel, and that such data could be used to estimate the relatedness between taxa. It was also pointed out in the preliminary paper that the interval required to competitively equalize "foreign" and "domestic" pollen in double pollinaI Received for publication 18 February 1970. The author was the recipient of NSF grant GB-6854 during this study. tions would be useful as an estimate of the frequency of hybridization in nature, since the longer the interval the less the probability of hybridization. This paper is a report of the completed study on pollen competition in section Isopappus. MIATERIALS AND METHODS-MIature plants were grown in the greenhouse from seeds collected in Kansas and Texas (Smith, 1968a). Crosses were made on sunny or partly-cloudy days, in the usual manner, with lens-paper bags utilized to prevent open pollination. All individuals of the taxa in the section are self-incompatible. The crosses were made so that the potency of competing pollen could be equalized, as indicated by 50 % of hybrids in the progeny. Through double pollination, with the slower-growing pollen used first, followed (after a delay) by the fastergrowing pollen, the pollen types can be matched in potency. The crosses may be represented: domestic (stigma) X foreign (pollen)X Domestic (pollen), the dash indicating the timed delay. The length of the delay was varied from 0 min (simple double pollination) to as much as 180 min in some crosses involving Haplopappus rigidifolius. The seeds from crosses were planted in the greenhouse2 the progeny raised to maturity, and the pollen stainability of the progeny tested in propionic acid-aniline blue (Smith, 1965). Hybrids in the section exhibit pollen stainability ranging from about 1-48% and are thus easily differentiated from normal plants, which have 97-100% pollen stainability. Crosses that produced either 0% or 100% hybrids were excluded from the results, since these could occur due to S-allele action (two of the three parents involved in the cross sharing the same S-alleles), unless it was known from previous crosses that the parental individuals would cross.