Seed Set In Crosses Research Articles

Evidence of Three Subspecies in Trifolium nigrescens Viv.

Germplasm accessions of Trifolium nigrescens Viv. were found to belong to three distinct taxa which differed in morphology, hybrid seed-set in crosses among themselves and with white clover (T. repens L.), chromosomal distribution of rDNA genes, and rDNA internally transcribed spacer (ITS) region sequences. These taxonomic groups correspond with T. nigrescens ssp. nigrescens, T. nigrescens ssp. petrisavii var.petrisavii and var. meneghinianum. The existence of var. meneghinianum has been rejected or ignored in recent taxonomic treatments. The present study indicates that it is very distinctive and warrants at least subspecies status. Phylogenetic analysis based on ITS sequences and pod morphology placed ssp. nigrescens as progenitor to the others. ITS sequences placed white clover closer to ssp. petrisavii but neither DNA sequences nor rDNA chromosome locations clearly showed whether var. petrisavii or var. meneghinianum was closer to white clover. In contrast, results of hybridization studies showed that ssp. nigrescens gave full seed-set when pollinated with T. repens whereas both ssp. petrisavii var. petrisavii and ssp. petrisavii var. meneghinianum gave poor seed-set. All three taxa showed close relationships with white clover and none can clearly be shown to be a direct ancestor of T. repens.

Self-Sterility in Epilobium obcordatum (Onagraceae)

Greenhouse pollinations among 23 field-collected plants and their progeny revealed a high and variable degree of self-sterility in Epilobium obcordatum. Field-collected plants averaged 5% seed-set when selfed and 43% seed-set in crosses; greenhouse progeny 11% and 49%, respectively. In two of three pollen chase tests, previously or simultaneously applied self-pollen reduced seed-set to that resulting from pure self-pollination. Based on the relative survivorship of self- and outcross embryos, the mean number of lethal equivalents was determined to be 11.0 (range 2.9 to 17.6) in the field-collected plants, and 7.1 (range 0.62 to 21.8) in the greenhouse progeny. Pollinations among 91 individuals showed a significant negative correlation between seed-set and inbreeding coefficients of the resulting zygotes. Intrapopulation crosses yielded significantly fewer seeds than interpopulation crosses. Early embryonic mutational load, in combination with some level of biparental inbreeding, appears to be responsible for the level of self-sterility in natural populations of this species. A pollen dosage test for an interpopulation cross resulted in a maximum seed set of 47% at 16 tetrads. Additional unmeasured characteristics govern the probability of ovules becoming mature seeds.

Artificial Interspecific Hybridization in Temperate and Tropical Species of Ruellia (Acanthaceae)

Artificial hybridization experiments involving 25 taxa ofRuellia had as one objective to determine the possibility of any genetic relationships between North American temperate species and certain widespread tropical American taxa. The results confirm a close morpho-genetic link betweenR. caroliniensis-R. pedunculata and the Caribbean species R.geminiflora and betweenR. nudiflora and the pantropical R.tuberosa. A second objective was to test the genetic basis for the present generic concept ofRuellia. The results of hybridizations among species representatives of six proposed segregate genera were measured by relative seed-set in crosses. The results support the view that the present circumscription of the genus is unnatural and that some taxa presently classified inRuellia bear little if any morphological or genetic relationship toRuellia. When the ecotype concept is applied to the genus, natural, closely related species groups can be identified by hybridization experiments. This approach can lead to an accurate, precise delimitation ofRuellia and may be applicable in other large genera of Acanthaceae.

Embryo Development and Seed Set in Crosses for Triploid Day Lilies

Only 18.6% of 1,607 pollinations between diploid and tetraploid Hemerocallis formed capsules that survived beyond 35 days. Most seeds were aborted and nonviable. Twenty-nine triploid seedlings were obtained, 20 from tetraploid seed parents and nine from diploid seed parents. Most triploids were from crosses of Shirred Apricot and Imperial Blush with various seedlings of Tetra Chetco × Tetra Sherwood, indicating possible genetic influences on the survival of triploid zygotes. In normal seeds the endosperm and ovule grew rapidly during the early developmental stages, attaining near maximum sizes by the end of the third and fourth weeks, respectively. Embryos remained relatively small until the fourth week and attained mature size by the fifth week. Most triploid embryos aborted within the first 2 weeks.