Cytogenetics of Lathyrus palustris, a natural autohexaploid

American as well as British forms of the perennial plant Lathyrus palustris have 2n = 42 chromosomes with one group of 6 long submetacentric chromosomes, two groups of 6 medium-long subacrocentric chromosomes, three groups of 6 medium-long to medium-short submetacentric to subacrocentric chromosomes, and one group of 6 medium-short metacentric chromosomes. One haploid complement measures 45.8 μm at somatic metaphase. At meiotic first metaphase both forms show extensive multivalent formation. In 50 metaphase I cells of one plant of the British form there were 51 ring hexavalents, 80 chain and open-branched hexavalents of various forms, 2 quinquivalents, 41 ring quadrivalents, 58 chain quadrivalents, 20 branched quadrivalents of various forms, 1 trivalent, 128 ring bivalents, 280 open bivalents, and 9 univalents. It is concluded that L. palustris is most likely a natural autohexaploid with frequent multivalent pairing, many points of pairing initiation, several partner exchanges per multivalent, and many interstitial chiasmata. It may well be the first fully documented true natural autohexaploid reported. Pollen fertility was 56.2% and seed set 37%, apparently sufficient for a perennial plant species.Key words: Lathyrus, natural, autohexaploid, meiosis, multivalents.

Origin of B chromosomes in cultivated rye

Cultivated rye (Secale cereale) and its weedy relative (S. segetale) carry B chromosomes. The B chromosomes are known to be morphologically alike at somatic metaphase and they are of the standard type in natural populations. To clarify the cytogenetic relationship between the standard B chromosomes of S. cereale and those of S. segetale, we made four crosses between Afghan S. segetale with two standard B chromosomes as a pistillate parent and Turkish, Iranian, Korean, and Japanese S. cereale, all with two standard B chromosomes as pollen parents. We observed the pairing of B chromosomes at diakinesis in pollen mother cells in all F1 hybrids with four standard B chromosomes, two from each of the pistillate and the pollen parents. The degree of pairing of B chromosomes in all F1 hybrids with four standard B chromosomes was similar to or somewhat lower than, that in parental strains with four standard B chromosomes. These results showed that the standard B chromosomes in S. segetale from Afghanistan are homologous with those in S. cereale from Turkey, Iran, Korea, and Japan. We therefore propose monophyletic origin of the standard B chromosomes in S. segetale and S. cereale.Key words: Secale, B chromosomes, origin, cultivated rye, weedy rye.

Occurrence of G-banding in metaphase chromosomes of Encarsia berlesei (Hymenoptera: Aphelinidae)

Well-defined G-bands were obtained on somatic metaphase chromosomes of Encarsia berlesei using trypsin and warm 2× SCC in sequence. The G-banded pattern allowed rapid identification of all five metacentric chromosomes, which appeared uniformly lighted when stained with DAPI fluorochrome dye. It is stressed that ageing affects G-banding in this insect species; in fact, good banded chromosomes were obtained on 1-month air-stored chromosomes. Evidence for asynchronous condensation on the chromosomes of this species is also provided.Key words: G-banding, chromosomes, insect, Encarsia.

In situ localization of two repetitive DNA sequences to surface-spread pachytene chromosomes of rye

Surface-spread pollen mother cells at meiotic prophase from Secale cereale (rye) were used for fluorescent DNA:DNA in situ localization of two tandemly repeated DNA sequences: pTa71, a wheat rDNA clone, and pSc119.2, a cloned 120-bp repeat from rye heterochromatin. The fluorescent hybridization signal, consisting of many yellow-green dots, was closely associated with the bivalent axes, corresponding to the synaptonemal complex, and located in the surrounding chromatin. The rDNA signal was associated with one bivalent, the smallest of the seven, at a distance about 13% of the bivalent length from the telomere. This corresponded to the position of the nucleolar organizing region of silver-stained synaptonemal complexes analyzed under the electron microscope and published data for somatic metaphase chromosomes. The relative length of the axis covered with the rDNA signal is less than expected from somatic metaphases, but it corresponds more closely to the proportion of the sequences in the genome. The hybridization signal with the 120-bp repeat was located mainly at the telomeric regions of several bivalents that showed thickenings of the axis after DAPI staining, probably corresponding to somatic C-bands. These major and some minor intercalary sites agree with the distribution of the 120-bp repeat in somatic metaphase. Fluorescent in situ hybridization to plant surface-spread pachytene chromosomes, which can be obtained in large numbers, has great potential for studying meiotic prophase, high-resolution mapping of DNA sequences, and investigating the relationship of DNA sequences to the synaptonemal complex.Key words: in situ hybridization, cereals, pachytene, meiosis, synaptonemal complex, physical mapping.