The genus Issatchenkia Kudriavzev has been emended to include all nitratenegative, multilateral budding yeast species that form unconjugated persistent asci with roughened spheroidal ascospores and have Q-7 ubiquinone in the electron transport system. Pichia kudriauzeuii (Issatchenkia orientalis), Pichia terricola, Pichia scutulata var. scutulata, and Pichia scutulata var. exigua are assigned to this genus as Issatchenkia orientalis Kudriavzev exigua showed only 25% deoxyribonucleic acid complementarity, yet intervarietal matings formed viable ascospores. This is the lowest deoxyribonucleic acid relatedness ever shown between strains capable of genetic hybridization, and the implications of this finding are discussed. The genus Issatchenkia was described by Kudriavzev (11) in 1960 for yeasts which were isolated from fruit juices and berries and which had unconjugated asci and spheroidal ascospores. The single species originally included in the genus, Issatchenkia orientalis Kudriavzev Pichia terricola van der Walt and Pichia scutulata PhafT, Miller et Miranda are two other taxa phenotypically similar to P. kudriauzeuii. Scanning electron microscopy has shown that all three have warty ascospore walls (14,23), and in this respect, they differ from other roundspored species of Pichia, which have essentially smooth spore walls. Although these three species are similar in ascospore surface structure to species of Torulaspora (van der Walt [31] group 111, Saccharomyces species) and Debaryomyces (17), they do not form the asci with tapered projections that are associated with Torulaspora or exhibit the mother-daughter conjugation typical of Debaryomyces. Consequently, when ascus morphology and ascospore ultrastructure are considered, these species appear to represent a natural grouping that is different from other yeast genera, and we propose to place them in the genus Issatchenkia. On the basis of deoxyribonucleic acid (DNA) base sequence complementarity, mating reaction, and ascospore ultrastructure, we describe one new species, and several suggested perfect-imperfect relationships are verif'ied. Additionally, our multifaceted approach allowed an examination of species parameters that heretofore was not possible and suggested that, as is the case among some higher eucaryotes, the boundaries of individual yeast species may be less precisely definable than previously supposed. 503
Two new species of the yeast genus Pichia were isolated from frass obtained from loblolly pines growing in Mississippi. The new taxa, designated Pichia amylophila and Pichia mississippiensis, are heterothallic, and Candida o btusa var. arabinosa proved to be a haploid member of the latter species. The species show interspecific mating, but ascospores from the crosses are infertile. The nuclear deoxyribonucleic acids from the species differ by 2 mol% in guanine plus cytosine content and exhibit low (25%) base sequence complementarity. Deoxyribonucleic acid relatedness was determined spectrophotometrically, and the methodology is given in detail.During the isolation of yeasts from insect frass collected from pine trees in Mississippi, we recovered strains that were phenotypically similar to Pichia wickerhamii (van der Walt) Kregervan Rij and Pichia rhodanensis (Ramirez et Boidin) Phaff. These strains were heterothallic but failed to conjugate with mating types from either P. wickerhamii or P. rhodanensis. One of the newly isolated cultures differed from the others by its ability to assimilate starch, and genetic crosses between it and the other isolates gave only infertile and generally poorly formed ascospores. In an effort to clarify speciation, we examined the nuclear deoxyribonucleic acid (DNA) base sequence complementarity of the strains. As a result, the isolates proved to be of particular interest because they allowed an examination of species parameters through both genetic crosses and DNA reassociation studies. ilation tests were previously given (21). Single-ascospore isolates were obtained by micromanipulation. MATERIALS AND METHODSThe criteria used for speciation within the genus Pichia were those proposed by Kreger-van Rij (7).DNA purification and determination of base composition. Extraction and purification of DNA were done by a combination of the procedures of Marmur (9) and A~~o / A~w were used to assess DNA purity. Purification was repeated if the preparation deviated more than 0.05 from the ratios AZW/A'W = 1.86 and ABO/AM = 0.5 (11). The quality of the DNA was further assessed from analytical ultracentrifuge scans and from thermal-melt profiles. Hyperchromicity of the preparations ranged from 34.4 to 36.8%.The guanine plus cytosine (G+C) content of the nuclear DNA was calculated from buoyant density values in cesium chloride (16,19) and was based on three or four separate determinations made with a Spinco model E analytical ultracentrifuge equipped with an electronic scanner. Micrococcus lysodeikticus Fleming DNA was used as a reference; this DNA had a buoyant density of 1.7311 g/ml when compared to DNA from Escherichia coli K-12, whose density was taken to be 1.7100 g/ml (16). DNA reannealing reactions.The extent of DNA reassociation was determined spectrophotometrically, essentially by the method reported by Seidler and Mandel(18) and Seidler et al. (17), as described below. DNA was sheared by two passages through a French press at 10, OOO lb/in2 and was recovered as intact double-strand...
The maintenance of structural integrity in the DNA of aging mice has been examined with the amin in view of determining whether changes in genome structure constitute the molecular basis of aging. Cell lysate DNA from brains of differently aged mice was subjected to alkaline sucrose gradient sedimentation. The results show that brain DNA from young mice sediments mondispersely while that from senescent mice exhibits polydisperse sedimentation patterns, bainding in four peaks corresponding to number-average molecular weights of 1.4-10(8), 70-10(6), 15-10(6) and 3-10(6). When treated with nuclease S1, it was the 30 month mouse DNA whose sedimentation shifted to the top of the gradient indicating a reduction in its molecular weight as a result of nuclease digestion. The apparent increase in single strand breaks implies that the rate of breakage in old mouse brain DNA is faster than that of repair replication. The conclusion is drawn that senescence could result from an accumulation of defects in the genome.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.