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Maureen C. O'Leary and Thomas H. Boyle

Isozyme markers were used to identify cultivars and assess the genetic diversity within a germplasm collection of 49 Hatiora Britt. & Rose clones. The collection included accessions of Easter cactus [H. gaertneri (Regel) Barthlott, H. graeseri Barthlott ex D. Hunt, and H. rosea (Lagerheim) Barthlott] plus H. herminiae (Campos-Porto & Castellanos) Backeberg ex Barthlott and H. salcornioides (Haworth) Britton & Rose. Seven enzyme systems were analyzed: aspartate aminotransferase, glucose-6-phosphate isomerase, leucine aminopeptidase, malate dehydrogenase, phosphoglucomutase, shikimate dehydrogenase, and triosephosphate isomerase. Thirteen loci and 42 alleles were identified. Twenty-one clones (43%) displayed unique isozyme profiles, but the remaining 28 clones shared isozyme profiles with one to three other clones. Percent polymorphic loci, mean number of alleles per locus, and mean heterozygosity were 69, 3.23, and 0.30, respectively, for the entire collection. Isozymes also proved useful for verifying that some progeny were genuine F1 hybrids.

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Maureen C. O'Leary and Thomas H. Boyle

Cultivars and seedlings of Rhipsalidopsis and Schlumbergera were subjected to isozyme analysis using seven enzyme systems [aspartate aminotransferase (AAT), aminopeptidase (AMP), glucose-6-phosphate isomerase (GPI), malate dehydrogenase (MDH), phosphoglucomutase (PGM), shikimate dehydrogenase (SKD), and triose phosphate isomerase (TPI)]. Isozymes were extracted from phylloclades and roots, and were separated by polyacrylamide gel electrophoresis (PAGE) using single percentage (5% to 10%) gels. Six enzymes exhibited polymorphism in Rhipsalidopsis, whereas all seven enzymes were polymorphic in Schlumbergera. Inheritance studies were performed on AAT, GPI, MDH, PGM, and TPI for Rhipsalidopsis and on AMP, PGM, and SKD for Schlumbergera. Significant segregation distortion was observed in some families. Polymorphic isozymes are potentially useful markers for cultivar identification and for genetic and breeding studies.

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Maureen C. O'Leary and Thomas H. Boyle

A germplasm collection of 59 Schlumbergera Lemaire clones was assayed for isozymes of aspartate aminotransferase, glucose-6-phosphate isomerase, leucine aminopeptidase, malate dehydrogenase, phosphoglucomutase, shikimate dehydrogenase, and triosephosphate isomerase. The collection included cultivars of holiday cactus [S. truncata (Haworth) Moran and S. ×buckleyi (T. Moore) Tjaden] plus accessions of S. kautskyi (Horobin & McMillan) N.P. Taylor, S. opuntioides (Löfgren & Dusén) D. Hunt, S. orssichiana Barthlott & McMillan, S. russelliana (Hooker) Britton & Rose, S. ×exotica Barthlott & Rauh, and S. ×reginae McMillan & Orssich. Twelve loci with 36 alleles were detected. Percent polymorphic loci, mean number of alleles per locus, and mean heterozygosity were 83, 3.00, and 0.24, respectively, for the entire collection. Forty-one clones (69%) could be distinguished solely on the basis of their isozyme profiles, but the remaining 18 clones shared profiles with one or two other clones. Isozymes proved useful for determining the parentage of some clones and verifying that some progeny were interspecific hybrids. About 28% of the genetic diversity within the entire collection is unique to four Schlumbergera species that have scarcely been exploited for breeding holiday cactus cultivars.

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Maureen C. O'Leary and Thomas H. Boyle

Polyacrylamide gel electrophoresis was used to study inheritance and linkage of isozymes in Easter cactus (Hatiora species and interspecific hybrids). Five isozyme systems were analyzed: aspartate aminotransferase (AAT), glucose-6-phosphate isomerase (GPI), malate dehydrogenase (MDH), phosphoglucomutase (PGM), and triosephosphate isomerase (TPI). F1, F2, BC1, and S1 progeny were used for inheritance studies. Six polymorphic loci (Aat-1, Gpi-1, Mdh-1, Pgm-1, Pgm-2, and Tpi-2) were identified. Aat-1 and Pgm-1 were linked (recombination frequency = 26% ± 7%), but the other isozyme loci assorted independently. Aberrant segregation ratios were observed in at least one segregating family for all six isozyme loci. We hypothesize that segregation distortion was due to linkage between isozyme loci and other genes subject to pre- or postzygotic selection. The existence of five additional isozyme loci (Aat-2, Gpi-2, Mdh-2, Mdh-3, and Tpi-1) was inferred from segregation patterns and by comparison of isozyme profiles from phylloclades and pollen. These isozyme loci may prove useful for confirming hybridity in intra- and interspecific crosses, determining parentage of cultivars, and assessing genetic diversity in germplasm collections.

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Thomas H. Boyle, Fabian D. Menalled and Maureen C. O'Leary

The existence of self-incompatibility (SI) was demonstrated in `Britton' and `Rose' Easter cactus (Rhipsalidopsis). In a full diallel cross among five clones, 18 out of 20 outcrosses resulted in 68% to 100% fruit set, whereas reciprocal crosses between two of the clones and all five self-pollinations failed to set fruit. Pollen tube growth was greatly inhibited in styles of selfed pistils, but there was no evidence of pollen tube inhibition in compatibly crossed pistils. Easter cactus exhibited characteristics typically found in sporophytic SI systems (trinucleate pollen, papillate stigmas, and scant stigmatic exudate) together with those associated with gametophytic SI systems (stylar inhibition of pollen tube growth and absence of reciprocal differences in outcrosses). Additional experiments were performed to determine the effects of bud pollinations, growth regulators (BA, GA3, and NAAm), and high temperatures (0- to 48-h exposure at 40C) on the SI response. Heat treatments were more effective than either bud pollinations or growth regulators in overcoming SI, and yielded an average of 7.2 viable seeds per treated flower when plants were incubated for 12 h at 40C and selfed immediately after incubation. Isozyme analysis of the S0 parent and putative S1 progeny confirmed that selfing had occurred following heat treatments. Using S1 progeny in breeding programs may extend the flower color range and lead to a greater diversity in other plant characteristics than presently exists in cultivated germplasm. Chemical names used: N-(phenylmethyl)-1H-purin-6-amine [benzyladenine (BA)], gibberellic acid (GA3), and α-naphthaleneacetamide (NAAm).

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Renate Karle, Constance A. Parks, Maureen C. O'Leary and Thomas H. Boyle

Spontaneous chromosome doubling occurred in shoot apices of two diploid (2n = 22) Hatiora ×graeseri Barthlott ex D. Hunt (Easter cactus) clones and yielded stable periclinal cytochimeras with a diploid epidermis and tetraploid subepidermis. The cytochimeras produced disomic gametes (n = 22) and displayed tetrasomic inheritance at polymorphic isozyme loci. Diploid clones were highly self-incompatible (SI) but both cytochimeras were self-compatible (SC). Analysis of pollen tube growth in selfed or outcrossed styles revealed that polyploidy altered the incompatibility phenotype of pollen without affecting the incompatibility phenotype of the pistil. Morphological data (guard cell length, stomatal density, and pollen diameter), segregation ratios at isozyme loci, and fruit/seed yields indicate that S1 progeny are SC, nonchimeral, and tetraploid. Breakdown of the SI system in the cytochimeras was attributed to formation of compatible heteroallelic pollen. These results provide a rational explanation for the correlation between ploidy level and breeding behavior in cacti. Production of SC autotetraploid clones from SI diploids by chromosome doubling may be useful in development of cacti as fruit crops.