. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. MEIOTIC CROSSOVER IN A MOLLUSC 147. Figure 1. Chiasma formation in eggs of the dwarf surfclam, Muli- nia lateralis. showing "V" or "X" shaped bivalents with only one chi- asma. If there were two or three crossovers per chromosome, bivalents would have appeared in the shapes of "O" or "8" respectively. The single chiasma usually formed near the centromere ("V" shaped), ex- cept for 2 or 3 bivalents. Eggs were fixed in 3:1 methanol and acetic acid and stained with an acetic orc
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. MEIOTIC CROSSOVER IN A MOLLUSC 147. Figure 1. Chiasma formation in eggs of the dwarf surfclam, Muli- nia lateralis. showing "V" or "X" shaped bivalents with only one chi- asma. If there were two or three crossovers per chromosome, bivalents would have appeared in the shapes of "O" or "8" respectively. The single chiasma usually formed near the centromere ("V" shaped), ex- cept for 2 or 3 bivalents. Eggs were fixed in 3:1 methanol and acetic acid and stained with an acetic orcein stain. by 6% with Kosambi's map function and by 28% with Haldane's function, and the inflation increases dramati- cally for longer distances. The agreement between the finding of one crossover per chromosome through segregation analysis (at six loci) and the cytogenetic observation of one chiasma per chromosome weakens the chiasma reduction hypothe- sis. The chiasma reduction hypothesis states that some chiasmata may be eliminated prior to metaphase, and the observed chiasma frequency is an underestimate of actual recombination events, causing the chiasma-based maps to be shorter than recombination-based maps (18, 19). Our results indicate that there was no chiasma re- duction—one recombination corresponds directly to one chiasma—for at least four of the 19 chromosomes in A/, latcralis (AAT, GPI, ALAT2 are linked). For four other chromosomes, chiasma reduction, if any, must be small, because the chance that two recombinations oc- curred is less than 5% according to the CR values. It is true that recombination maps are usually longer than chiasma maps (18, 20). This discrepancy is likely caused by the inflation of the linkage map due to the underesti- mation of interference as discussed earlier, and not by chiasma reduction. Another unexpected finding was that the CR values of the 13 loci seem not to be randomly distributed between Oand 100 (Table I). Ten of the 13 loci
Size: 1486px × 1682px
Photo credit: © Library Book Collection / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookauthorlilliefrankrat, booksubjectbiology, booksubjectzoology
