Eight highbush blueberry (V. corymbosum L.) triploids (2n = 3x = 36) were crossed with diploids (2n = 2x = 24), tetraploids (2n = 4x = 48), and hexaploids (2n = 6x = 72). No plants were recovered from 4021 3x × 2x crosses. One triploid was relatively fertile in 3x × 4x and 3x × 6x crosses, which is most likely attributable to 2n gamete production in the triploid. The lack of fertility of triploids, which do not produce 2n gametes, in crosses with diploids and tetraploids suggests that the production of gametes with numerically balanced (n = 12 or 24) chromosome numbers is extremely low. In addition, the inability to recover progeny from 3x × 2x crosses also suggests that aneuploid gametophytes and/or zygotes, including trisomics, are inviable in blueberry. Pollen stainability was also highly reduced in triploids. Frequency distributions of anaphase I pole chromosomal constitutions of three triploids were significantly different from one another. Two of the three distributions were shifted toward the basic chromosome number of 12, with one triploid having 25% poles with 12 chromosomes. However, the sterility of 3x × 2x and 2x × 3x crosses indicates that lagging chromosomes during meiotic anaphases are probably not excluded from gametes, resulting in unbalanced gametes in blueberry. Triploids can be used as a bridge to facilitate gene transfer from the diploid and tetraploid levels to the hexaploid level in blueberry.
N. Vorsa and James R. Ballington
Ryan N. Contreras, Thomas G. Ranney, and Shyamalrau P. Tallury
applied, and cells observed at metaphase I and anaphase I using a light microscope (Carl Zeiss photomicroscope; Carl Zeiss MicroImaging, Inc., Thornwood, N.Y.) under ×600 and ×1000 magnification. Assessing pollen fertility. Pollen fertility was
Chunsheng Lu and Mark Bridgen
Self-pollinations of a diploid (2n = 2x = 16) interspecific hybrid from the cross of Alstroemeria aurea × A. caryophyllaea resulted in no seed set. Pollen viability studies with the hybrid demonstrated that only 5% of the pollen grains were viable. Cytological observations with the hybrid pollen mother cell (PMC) revealed abnormal chromosome behaviors, such as no pairing in Prophase I and Metaphase I, and bridges in the Anaphase I and II. Although the development of microspores appeared normal in shape until the stage of tetrad release, some chromosomes did not remain in the nucleus after completing meiosis, formed isolated groups of 1 to 4, and remained in the cytoplasm. This genetic imbalance of the microspores could be one of the causes for the abortion of the pollen grains in the late stage of development. Additional meiotic cytological studies with colchicine-induced tetraploids (2n = 4x = 32) derived from the hybrid plants showed that chromosome pairings were normal in most cases. However, self-pollination with the tetraploid plants failed to set seeds. These studies with the tetraploids further demonstrate that the sterility of the hybrid is due not only to chromosomal differences, but also to complex genic interactions.
John R. Stommel
Solanum ochranthum Dunal is a nontuber bearing wild relative of the cultivated tomato (Lycopersicon esculentum Mill.), and a potential source of new genes for disease and pest resistance. Because S. ochranthum is sexually isolated from tomato, somatic hybrids between tomato (PI 367942; L. esculentum Mill. var. cerasiforme (Dunal) A. Gray VFNT cherry × L. peruvianum (L.) Mill. backcrossed to VFNT cherry) and S. ochranthum (LA2117) were developed previously to overcome these crossing barriers. Attempts to backcross these hybrids to tomato have been unsuccessful. Pollen fertility and mitotic and meiotic studies in tomato + S. ochranthum somatic hybrids determined the cause of the sterility of the somatic hybrids and identified hybrids with moderate fertility. Chromosome counts of dividing root tip cells delineated tetraploid (2n = 4x = 48) and hexaploid (2n = 6x = 72) genotypes and aneuploidy in these hybrids. Meiotic analysis of developing microspores confirmed the presence of precocious division and laggard chromosomes at anaphase in both hexaploid and tetraploid hybrids. Bridges were observed in hexaploids at anaphase I and II and multivalent configurations were observed at diakinesis. Multivalents and univalents were evident in nearly all cells examined, proving that the two genomes are homoeologous. Aberrant microsporocytes with five to six developing microspores were noted in hexaploid hybrids. The occurrence of homoeologous pairing between chromosomes of both fusion parents is advantageous to effect recombination between these isolated species. However, the negative effects of multivalent formation and univalents likely contributed to observed sterility in these first generation fusion hybrids. Low to moderate levels of pollen fertility (0% to 52%) were found in tetraploid hybrids, while little or no viable pollen (0% to 4%) was observed in hexaploid somatic hybrids.
Jessica Gaus Barb, Dennis J. Werner, and Shyamalrau P. Tallury
, respectively ( Fig. 4 , data not shown). Lagging chromosomes (one to five) were present in either Anaphase I or Anaphase II in 53% of the 34 PMCs observed ( Fig. 5 ). Disjunction at Anaphase I and II in triploid cells without lagging chromosomes produced nearly
Pablo Bolaños-Villegas, Shih-Wen Chin, and Fure-Chyi Chen
anaphase I depends on prior pairing, synapsis, and recombination at earlier stages ( Armstrong and Jones, 2003 ; Armstrong et al., 2001 ). Nevertheless, chromosome recognition and initial pairing in eukaryotes is not achieved by DNA–DNA interactions alone
David M. Czarnecki II and Zhanao Deng
/or not separating to opposite poles during anaphase I. In SDR, the second meiotic division occurs abnormally, with sister chromatids not separating to opposite poles during anaphase II. It has been shown that univalents and multivalents (during prophase I
Brian M. Schwartz, Ryan N. Contreras, Karen R. Harris-Shultz, Douglas L. Heckart, Jason B. Peake, and Paul L. Raymer
metaphase I or anaphase I were observed to determine the chromosome number. The somatic chromosome numbers of ‘Sea Spray’, ‘SeaStar’, and 11-TSP-1 were also determined using root tip squashes collected from plants grown in pots filled with perlite. After a
Xuhong Zhou, Xijun Mo, Yalian Jiang, Hao Zhang, Rongpei Yu, Lihua Wang, Jihua Wang, and Suping Qu
. Following chromosome condensation and crossover formation in pachytene and diakinesis ( Fig. 1C and D ), the metaphase I spindle aligns the bivalents at the equatorial plane ( Fig. 1E ) and subsequently, segregates the homologs toward the poles at anaphase I
Yi-Lu Jiang, Tzong-Shyan Lin, Ching-Lung Lee, Chung-Ruey Yen, and Wen-Ju Yang
. Yellow pitaya is a tetraploid (2n = 44) in which meiosis occurs irregularly at anaphase I and therefore often results in reduced pollen viability. Low pollen viability causes low seed numbers per fruit even when fruit set is not a problem in self