Cytogenetic studies involving eighteen species of Potentilla of the section Gymnocarpae were made to investigate their phylogenetic relationships and to see whether any correlation exists between morphology and crossing relationships.The chromosome numbers of Potentilla ambigens Greene {zn = 82) and P. montenegrina Pant, {zn = 28) were determined for the first time. New intraspecific chromosome races were observed in three species: P. pennsylvanica L. (2« = 42), P. astrisanguinea Lodd. {zn = 84) and P. grandiflora L. (2« = 42).Hybrids were obtained from only four of 628 crosses in iii different combinations (P. nepalensis Hook, x P. montenegrina Pant., P. astrisanguinea Lodd. x P. ambigens Greene, P recta L. x P. cf. hirta L. and P. grandiflora L. x P. crantzii (Crantz) Beck.). The position of the crossing barrier in the unsuccessful crosses varied. Sometimes the pollen failed to germinate but in other cases well-developed pollen tubes were observed. No definite correlation was found between the morphological similarity of species and their crossing ability.Meiotic studies on the hybrids indicated some chromosomal homologies between the genomes of P. recta and P. cf. hirta and between P. grandiflora and P. crantzii, but not between P. nepalensis and P. montenegrina.
SUMMARY Three natural hybrids have been reported in Potentilla subseries Tormentillae: Potentilla Y. suberecta, P. ×mixta and P. × italica. This paper presents results of morphological and cyto‐logical studies and experimental crosses made to clarify the origins and taxonomy of the hybrids, and to investigate the possibility of introgression. All natural hybrids examined were hexaploid (2n= 42). Experimental crosses confirmed that P. × suberecta is the hybrid between P. erecta (2n= 28) and P. anglica (2n= 56), but the origin of the other hybrids is less clear. It is suggested that the names P. ×mixta and P. ×italica have been applied to different forms of a single variable taxon, and that P. × italica should be regarded as a synonym of P. ×mixta. The origin of this natural hybrid remains uncertain. It is generally considered to be P. reptans (2n= 28) ×P. anglica, but numerous attempts to hybridize these species have failed, although one unpublished, successful hybridization is cited. Experimental crosses have indicated an alternative origin for P. ×mixta, by hybridization between P. reptans and P. erecta, in which the former either contributes an unreduced gamete or occurs as an autopolyploid cytotype. Vigorous backcross progenies were raised from both P. ×suberecta and P. ×mixta and the importance of backcrossing and introgression in natural populations is discussed.
SUMMARYThree closely related pseudogamic species, Potentilla argentea L., P. recta L. and P. cf. hirta L. were pollinated by other species of Potentilla and observations were made on pollen germination, pollen tube growth, ovule development and seed set. The three pseudogamic species stimulated seed set in each other, but no seeds were formed after pollination by ten other species although well-developed pollen tubes were observed in a number of cases. The offspring from the successful pollinations were mainly maternal but a few deviant individuals were also produced.
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