Rates and other features of natural cross-pollination of Lycopersicon esculentum in stations within the range of wild forms in Perú were studied by counting the number of seeds and fruits produced by male-sterile mutants and were compared with similar data previously obtained in California, where no species of Lycopersicon are native. Useful data, which were provided by two plots, both located within the range of distribution of L. esculentum, L. pimpinellifolium, and L. peruvianum, showed much
higher rates of natural cross-pollination than were obtained in any tests outside the range of distribution. Mean values of 14.2 and 14.8 seeds per flower, construed to be the most useful single measure of cross-pollination, for the Peruvian plots contrast with numbers varying from 0.3 to 7.0 for 16 plots in California. Expressed as a proportion of the fertile-plant yield, the values for Peru are respectively 41.4 and 51.4 per cent and for California vary from one to 33 per cent. The interbranch and interplant variability is generally lower in the Peruvian than in the Californian data. The appearance of natural hybrids between varieties of L. esculentum and between this species and L. pimpinellifolium and also the much greater activity of insect pollen vectors observed in Peru lends support to the direct measures of natural cross-pollination. The insects visiting flowers of tomatoes were collected and the identifications by P. H. Timberlake are presented herewith. L. esculentum in the region studied is actively pollinated by two species of native solitary bees. Bees of nine other species representing diverse families were obtained in cursory collections from other species of Lycopersicon. The results are discussed in relation to (1) commercial production of F1 hybrid seed, (2) the change in breeding system during the historical transfer of L. escu-lentum to non-native areas from a facultatively cross-pollinated species to an almost exclusively self-pollinated one, and (3) the bearing of the change in pollination relations on flower form, heterosis, and appearance of mutants.
1. Gynodioecy-interbreeding hermaphrodites and females-is an outbreeding mechanism which differs from other systems in that the two mating types contribute unequal numbers of genes to the next generation. 2. Origanum vulgare has a gydodioecious system which is controlled by the segregation of two independent genes. The dominant gene F causes anther abortion and the gene H is a dominant suppressor of F. Females are therefore Ffhh or FFhh, hermaphrodites are FfHh, FfHH, ffHh or FFHH. 3. The bottom
recessive ffhh has not been found and is presumed to be lethal. The HH genotype arises in the proportion of one-quarter of the expected, and this genotype is self-incompatible. 4. The lethality and reduced production of these genotypes together with a lower seed output of the hermaphrodite are factors of importance in the maintenance of the two types in nature. 5. The genetic control in all its details supports a hypothesis that gynodioecy in Origanum has evolved from an incompatibility system with sporophytic pollen control and inhibition of fertilization in the embryo-sac.
The evolution of dimorphism in the pollen and stigmata of members of the Plumbaginaceae has been followed by a return, in some cases, to monomorphism. This has provided an extremely useful tool for the elucidation of genetical, genecological, phytogeographical and evolutionary problems. In the search for a comparable situation in other families, the neglected and ambiguous account of pollen dimorphism in 'Faramea' described by Fritz Müller and accepted by Charles Darwin has been reinvestigated.
Examination of the pollen of genuine Faramea, the tracing of Müller's herbarium material and the study of new collections has shown that Rudgea jasminoides (Cham.) Muell.-Arg. (Rubiaceae) is the species involved and that pollen dimorphism associated with heterostyly has evolved in the Rubiaceae in a manner quite comparable with that in the Plumbaginaceae. The difference between the two pollen forms of a single species of Rudgea is as great as that which distinguishes some genera or higher taxa yet it may have a simple genetical basis. Occurring as it does in a genus of woody tropical plants (and possibly being present in related genera with a similar habit), this dimorphism may be an important aid to the study of the history and distribution of tropical forest floras, of which there is little fossil record.
The alpine wildflower, Polemonium viscosum, depends on insect visitors for effective pollination. Here, I examine experimentally the effects of pollinator visitation on pollen removal, pollen dispersal success, paternity, and gene flow. Bumble bee pollinators visited donor individuals homozygous for marker alleles at an isozyme (GOT-2) encoding locus and then were presented with arrays of recipient plants lacking the marker alleles. Four aspects of male fitness were estimated for each donor: the
number of pollen grains dispersed to flowers of the first recipient visited, the number of offspring sired on that recipient, the proportion of offspring sired in the full array, and the proportion of mates in the array bearing seeds of the donor. Pollen removal was strongly influenced by the number of bee visits to donor flowers. The amount of pollen removed in turn significantly affected the number of pollen grains reaching flowers of the first recipient. However, because seed production decelerates with stigma pollen load, the relationship between pollen export and paternal success at this proximate scale showed diminishing returns. The probability of reaching mates within the array also increased with pollen export. These findings show that floral characters enhancing pollinator visitation rate in P. viscosum have positive effects on paternity and gene flow.
Pollination systems frequently reflect adaptations to particular groups of pollinators. Such systems are indicative of evolutionary specialization and have been important in angiosperm diversification. We studied the evolution of pollination systems in the large genus Ruellia. Phylogenetic analyses, morphological ordinations, ancestral state reconstructions, and a character mapping simulation were conducted to reveal key patterns in the direction and lability of floral characters associated with
pollination. We found significant floral morphological differences among species that were generally associated with different groups of floral visitors. Floral evolution has been highly labile and also directional. Some specialized systems such as hawkmoth or bat pollination are likely evolutionary dead-ends. In contrast, specialized pollination by hummingbirds is clearly not a dead-end. We found evidence for multiple reverse transitions from presumed ancestral hummingbird pollination to more derived bee or insect pollination. These repeated origins of insect pollination from hummingbird-pollinated ancestors have not evolved without historical baggage. Flowers of insect-pollinated species derived from hummingbird-pollinated ancestors are morphologically more similar to hummingbird flowers than they are to other more distantly related insect-pollinated flowers. Finally, some pollinator switches were concomitant with changes in floral morphology that are associated with those pollinators. These observations are consistent with the hypothesis that some transitions have been adaptive in the evolution of Ruellia.