A DNA cytophotometric and chromosome banding study in Hedera helix (Araliaceae), with reference to differential DNA replication associated with juvenile–adult phase change

The present DNA cytophotometric and chromosome banding comparison of the juvenile and the adult developmental phase in the common ivy, Hedera helix, was undertaken as an attempt to verify or refute the asserted phase-correlated differential DNA replication in this plant. No statistically significant differences between the two developmental phases were found in genome size (as determined from mitotic nuclei), nuclear DNA content in resting meristems and somatic leaf tissues, and content of constitutive heterochromatin as measured from C-banded metaphase plates. There is no evidence for differential DNA replication in Hedera helix. Key words: Hedera helix, differential DNA replication, morphogenesis, genome size, chromosome structure.

Qualitative changes in DNA indicating differential DNA replication during early embryogenesis of the newt Triturus vulgaris

During gastrulation of the newt Triturus vulgaris considerable changes in the meltingbehaviour and in the CsCl density gradient pattern of DNA occur. The melting curves of DNAs from mid to late gastrulae (stages 11 b-12c) deviate from the regular shape. Whereasthe Tm values are identical in the stages 11 a (early gastrula) and 15 (early neurula), and correspond to the standard DNA (stage 36 = tailbud), a significant rise of Tm (0·8–1·2 °C) has been recorded in the stages 12 a/b (yolk plug). The differences in melting behaviour become visible by the deviation of the curves above Tm. These deviations from the normal sigmoidal shape are caused by the fact that a portion of DNA melts at higher temperatures than usual. Therefore the thermal denaturation of DNA is completed at approximately 3–4 °C later than in standard DNA. Both the derivative curves and the plots on normal probability paper demonstrate a heterogeneity of DNA in the stages 11 b-12c which indicates the presence of an additional GC-rich satellite fraction. These findings are confirmed by CsCl density gradient studies. Thus, in the stages 12a/b a slight shoulder on the heavy side of the gradients occurs, being absent in the other stages. From these facts we have to conclude that there is a stage-dependent multiplication and elimination of GC-rich sequences.