Plastid Genome Size Research Articles

Nucleomorph genome diversity and its phylogenetic implications in cryptomonad algae

SUMMARY The relationship between phylogeny and nucleomorph genome size was examined in 16 strains of cryptomonad algae using pulsed-field gel electrophoresis, Southern hybridization and phylogenetic analyses. Our results suggest that all cryptomonads examined in this study contain three nucleomorph chromosomes and their total genome size ranges from 495 to 750 kb. In addition, we estimated the plastid genome size of the respective organisms. The plastid genomes of photosynthetic strains were approximately 120–160 kb in size, whereas the non-photosynthetic Cryptomonas paramecium NIES715 possesses a genome of approximately 70 kb. Phylogenetic analysis of the nuclear small subunit ribosomal DNA (SSU rDNA) gene showed that nucleomorph genome size varies considerably within closely related strains. This result indicates that the reduction of nucleomorph genomes is a rapid phenomenon that occurred multiple times independently during cryptomonad evolution. The nucleomorph genome sizes of Cryptomonas rostratiformis NIES277 appeared to be approximately 495 kb. This is smaller than that of Guillardia theta CCMP327, which until now was thought to have the smallest known nucleomorph genome size among photosynthetic cryptomonads.

Ultrastructural and molecular characterization of altered plastids in nuclear gene controlled yellow stripe mutant of Pennisetum americanum

The ultrastructural and molecular biological studies were conducted to elucidate the changes in the nuclear gene controlled plastid alterations in yellow stripe mutant of Pennisetum americanum. The plastids in yellow tissue were bound by a double membrane envelope and no internal thylakoid membrane differentiation, whereas plastid development was normal in the green tissue. The nuclear gene apparently influences the individual plastids as evidenced by the presence of heteroplastidic cells. Ribosomal RNAs were extracted from green and yellow seedlings with homozygous recessive and heterozygous genotypes. Plastid specific 23S and 16S rRNAs were absent in the yellow seedlings irrespective of the genotype. The presence of disorganized lamellar membranes in the rRNA/ribosome deficient plastids indicate that they are synthesized outside the plastid compartment. Restriction endonuclease analysis revealed no detectable differences in fragmentation pattern of the plastid DNAs from green and reverted green seedlings. This is consistent with the suggestion that the nuclear gene is not inducing any change in the plastid genome size and in the restriction enzyme recognition sites.

Plastid genome size and heterogeneous base composition of nuclear DNA from Ochrosphaera neapolitana (Prymnesiophyta)

We have studied the plastid genome of Ochrosphaera neapolitana (Hymenomonadaceae, Prymnesiophyta), a member of the chlorophyll a + c--containing algae. Total DNA from O. neapolitana was fractionated in a CsCl density gradient, and the ‘lighter’ upper band (which is typically of plastid origin) was unusually abundant relative to the ‘heavier’ nuclear one. Denaturation and renaturation experiments done with DNA isolated from the upper band showed that it had a base composition of 43.6 mol% G + C (molar fraction of guanosine plus cytosine), compared with 60.5 mol% G + C in the lower band, and that it lacked a substantial fraction of repetitive sequences; this DNA had a size of c. 10 Mb, which is more than an order of magnitude larger than has been reported for any plastid genome. Using pulse field gel electrophoresis (PFGE) of total DNA, we resolved a plastid genome size of approximately 165 kb for O. neapolitana. Although this is within the range reported for other plastid genomes, there is an increase of about 40 kb compared with related algae. Using the upper band from the CsCl gradient as a probe for the PFGE blots, we showed that this upper band is apparently a mixture of nuclear DNA with a small fraction of plastid DNA. The nuclear genome of O. neapolitana thus appears to have a distinctive heterogeneity in G + C content, with two DNA types differing by c. 17 mol%.