This file was created by scanning the printed publication. Errors identified by the software have been corrected; however, some errors may remain. CHAPTER 17 Summary

Major Conclusions Suggestions for Further Research 1. Arceuthobium is a well-defined, but morpho­ 1. The early studies of amino acid components logically reduced, of the , of dwarf and their host~ should be parasitic on and . The continued using contemporary molecular genus is probably of early Tertiary Period methods. origin in . Its closest relative is Notothixos, 2. Ultrastructural studies of the host-parasite which has a tropical Asian and Australasian interface should be expanded to gain a better distribution. understanding of these interactions. 2. Arceuthobium presumably migrated to the 3. Efforts to effect in vitro culture of dwarf New World in the pre-Miocene times. An mistletoes should be continued. intensive adaptive radiation occurred into the 4. The role of birds in long-distance dispersal is Pinaceae of the New World, and western poorly understood and requires additional is now the center of species study, especially in Arceuthobium occidentale diversity. Forty-six taxa are recognized in this and A. verticilliflorum. study, 38 of which are in the New World and 8 in the Old World. 5. Experimental studies to determine the cause of competitive host exclusion discussed in 3. Most of the Pinaceae of western North chapter 6 should be a fascinating area for America and Mexico have been colonized by future study. Arceuthobium. In the western United States, the hosts include at least some species of 6. The effective distance and mode of pollen Abies, Picea, Tsuga, and Larix, about 95% of transport should be investigated experimen­ the species of Pinus, and one of the two tally; pollen dispersal in Arceuthobium species of the Pseudotsuga. The few conifers rubrum should especially be examined. that are not known to be parasitized are pri­ 7. Chromosome numbers should be determined marily localized endemics. for the species not yet studied: Arceuthobium 4. Arceuthobium has a base chromosome num­ durangense, A. globosum subsp. globosum, ber of n = 14. No natural hybridization or A. guatemalense, A. rubrum, A. oaxacanum, polyploidy is known in the genus. The A. pendens, A. siskiyouense, A. tsugense (shore absence of hybridization and polyploidy has pine race),A. vaginatum subsp. vagina tum, apparently resulted in relatively clear, dendrit­ andA. yecorense and all the Old World ic lines of evolution and well-defined species. species (except A. oxycedri and A. juniperi­ procerae). Detailed analysis of the karyotypes 5. Most species are sympatric with other mem­ of related species groups, however, would be bers of the genus somewhere over their geo­ especially valuable. graphic distribution and the flowering periods of co-occurring species often overlap. The S. The possibility that sex chromosomes might species appear to be isolated reproductively. occur in and A. gillii should be studied. 6. The characteristics that delimit species, how­ ever, are often cryptic and may be apparent 9. A thorough analysiS of sexual dimorphism and for only short periods of the life cycle. Species sex ratio should be undertaken. and subspecies differ only in the number of 10. Crossing experiments should be carried out discontinuous variables, and with the excep­ between closely related species, e.g., tion of subspecies Arceuthobium apachecum-A. blumeri and A. vaginatum and cryptopodum in one area in campylopodum-A. occidentale. Chihuahua, even subspecies do not inter­ grade.

Summary 271 Chapter 17

11. Any aspects of the biology of the Asian species 16. The factors affecting the co-distribution of should be investigated. and Phora­ 12. The study of molecular systematics should be dendron juniperinum require investigation. continued. This offers a powerful methodolo­ 17. The ecological effects of dwarf infes­ gy to elucidate species relationships that are tation on stand structure, fire behavior, and often unclear in highly reduced groups (chap­ wildlife habitat should be studied. ter 15). 18. The selection, testing, and development of 13. The relationships and taxonomic status of sev­ genetic resistance should be expanded, both eral Mexican and Central American taxa from the basic understanding of resistance to require clarification: (1) the small of these parasites and as a practical ecological Arceuthobium globosum subsp. globosum approach to control. around El SaIto, Durango, in comparison with 19. The number of flower crops produced by the other populations of this subspecies in the Arceuthobium hawksworthii, A. abietis­ Sierra Madre Occidental, (2) the "stocky" pop­ religiosa, and A. juniperiprocerae should be ulations of A. rubrum in the vicinity of Altares, investigated, as well as the factors that limit the in northern Durango, (3) the occurrence of distribution of A. hawksworthii to the middle A. aureum subsp. petersonii in southern and upper altitudinal ranges of Pinus caribaea Oaxaca, (4) the identity of the dwarf mistletoe var. hondurensis in Belize. in El Salvador. 20. Effort to develop chemical controls that will 14. The northern limits of distribution for kill the endophytic system should continue. in Ontario and 21. The role of synergids in fertilization and early Quebec need to be established. embryo development need to be examined. 15. The unusual broom formation in species such 22. The effects of heat, smoke, and light on repro­ as Arceuthobium bicarinatum should be duction by dwarf mistletoe should be further studied. studied.

272 Summary