S H James

Cytoevolutionary patterns in Rutaceae

Chromosome numbers for 9 tribes and 73 genera ofRutaceae are examined for the probable chromosome base numbers in these taxa. There is abundant dysploidy and infrageneric polyploidy in the largeRutoideae/Toddalioideae complex. We found that x = 18 was typical for the tribesZanthoxyleae andToddalieae; probably ancestral in theBoronieae and perhaps in theRuteae, Diosmieae, andCusparieae; and characteristic of subfamilyFlindersioideae. Considering the basic position of elements ofZanthoxyleae andToddalieae in the family it appears that diploid x = 18 is ancestral in theRutaceae. The morphologically advancedCitroideae are invariant for x = 9 and may be a product of dysploid reduction.

The mating system and population genetic structure in a bird-pollinated mallee, Eucalyptus rhodantha

The mating system and spatial genetic structure of the rare and endangered bird-pollinated mallee Eucalyptus rhodantha were investigated in a remnant stand, using progeny arrays and pollen assayed at four polymorphic allozyme loci. Comparisons of the genetic diversity within and between the pollen pools and maternal parents of two arbitrary subpopulations indicated the presence of spatial genetic heterogeneity which was not broken down by pollen flow. It was suggested that this is the result of a high level if inbreeding and limited pollen dispersal by birds. Estimates of outcrossing rate ranged between [tcirc ]=0·59 and [tcirc ]=0·67 and were at the low end of the range reported for other eucalypts. It was concluded that E. rhodantha has a mixed mating system with a significant proportion of self-pollination. Biparental inbreeding within small neighbourhoods probably also contributed to the high level of inbreeding. The low level of outcrossing observed in E. rhodantha was not consistent with the hypothesis that bird pollination leads to high levels of outcrossing in the Australian flora. However, the level of outcrossing achieved through bird pollination together with high levels of gene flow between populations contribute to the maintenance of the relatively high levels of diversity which characterise the dissected populations of this species.

Genetic uniformity in Amphibolis antarctica , a dioecious seagrass

Few detailed studies have been published on genetic variation in seagrasses except those on the monoecious Zostera marina L. or the hermaphrodite Posidonia australis Hook. f. This paper presents allozyme, RFLP and reproductive biology data on Amphibolis antarctica (Labill.) Sonder&Aschers, one of the 75 per cent of all seagrass species which are dioecious. Collections were made from approximately one-third of the species range in Western Australia. Its only congener, A. griffithii (J. M. Black) den Hartog, was collected from one site to provide a comparison. Flowering was observed in 25 per cent of the shoots surveyed and the average sex ratio was 3.8: 1 (F:M) which it has been suggested indicates sexual reproduction. No genetic variation was found within or between populations at 14 allozyme loci. 18S RFLPs and M13 DNA fingerprinting gave few satisfactory results but also did not exhibit any variability. Allozyme variation was observed between A. antarctica and A. griffithii, the only congeneric species. The lack of allozyme and DNA variation within A. antarctica indicates a potentially low level of outbreeding, a highly clonal reproductive system or a very efficient genetic system in A. antarctica. The hypothesis that the dioecious reproductive system evolved in seagrasses to maximize outbreeding and genetic variability, proposed by several authors, is questioned in light of these data.

Complex hybridity in Isotoma petraea II. Components and operation of a possible evolutionary mechanism

Complex hybridity in Isotoma petraea II. Components and operation of a possible evolutionary mechanism

Genetic variation within and between populations of Posidonia australis , a hydrophilous, clonal seagrass

Allozyme diversity was surveyed at 15 loci across 22 populations of the hydrophilous seagrass Posidonia australis (Hook. f). Substantial genetic variation was detected (HT = 0.311) with a high proportion of this variation partitioned between populations (GST = 0.623). The high value of GST is attributed to large geographical distances between many of the populations and several of the extreme north-western populations having fixed homozygous genotypes. Southwestern populations of P. australis were the most variable and these correlate with the highest species diversity in this genus. Intermediate levels of genetic diversity are observed in P. australis when compared with other hydrophilous angiosperms. Average gene diversity values for hydrophilous taxa surveyed to date indicate lower HT and higher GST values than an average reported for 468 plant taxa. Patterns of genetic variability in different regions of the distribution of P. australis may reflect past evolutionary diversification into novel environments and subsequent dispersal following the rifting of Australia from Antarctica in the early Tertiary.

Genetic diversity in the cycad Macrozamia riedlei

The cycad, Macrozamia riedlei is a significant component of the native flora of southern Western Australia. It displays evolutionary conservatism not only in terms of its primitive morphology but also in the fact that only a single taxonomic species is recognized in this area. Microsporangia collected from 15 populations of the species were used to determine the extent and distribution of allozyme variation. The 14 loci studied showed high levels of polymorphism, an excess of heterozygotes relative to that expected under panmixia, some clinal variation but relatively low levels of interpopulational diversification. The level and distribution of diversity was similar to the average reported for conifers and angiosperm plants with similar life-history characteristics. These results contrast sharply with those reported in a similar study on M. communis, a cycad endemic to eastern Australia.

Complex hybridity in Isotoma petraea V. Allozyme variation and the pursuit of hybridity

SummaryIsoenzyme analysis of 12 structurally homozygous and 24 complex hybrid population samples of Isotoma petraea for ten enzyme systems revealed 9 polymorphic and 4 monomorphic loci. The structurally homozygous populations were relatively depauperate in allozyme heterozygotes so that rare alleles occurred more frequently as homozygous rather than heterozygous genotypes. The complex hybrid populations exhibited high levels of fixed allozymic hybridity and were 12·5 times as heterozygous at these loci as were the structural homozygotes. The distribution of allozymic variability supports the theory that complex hybridity in Isotoma originated on Pigeon Rock and migrated through the Isotoma population system to the south-west. Evidence was also adduced that during the evolution of the larger ringed complex hybrids, allozyme homozygosity sometimes replaced allozyme heterozygosity although the segments which carried these loci remained, indubitably, in fixed heterozygosity.

Chromosome numbers and their systematic implications in Australian marine angiosperms: ThePosidoniaceae

Somatic chromosome numbers of 2n = 20 are reported for all eight species of AustralianPosidonia: P. angustifolia, P. australis, P. coriacea, P. denhartogii, P. kirkmanii, P. ostenfeldii, P. robertsonae, andP. sinuosa. All species apparently have five larger and five smaller pairs of chromosomes. There is no evidence that speciation inPosidonia, which is relatively prolific in comparison to other seagrasses, is accompanied by change of chromosome number.

Temporal variation in allele frequencies in the pollen pool of Eucalyptus rhodantha

Investigation of the mallee Eucalyptus rhodantha which has an irregular, protracted flowering period demonstrated that individual plants differ in fecundity and phenology and that allele frequencies in the pollen pool change during the flowering season. Allele frequencies of the pollen pool were estimated by several methods which either assumed temporal homogeneity or took flowering phenology into account. These allele frequencies were compared for their abilities to estimate [tcirc ]. The estimates of [tcirc ] based on allele frequencies which took phenology into account were not consistently higher or better than those that did not at any of the four loci surveyed. Changes in the allele frequencies of the pollen pool during the flowering season may contribute to variation between some single-locus estimates of [tcirc ] in E. rhodantha which assume temporal homogeneity, but it was concluded that they are not the major cause of bias.

MOLECULAR PHYLOGENETIC ANALYSIS OF THE EVOLUTION OF COMPLEX HYBRIDITY IN ISOTOMA PETRAEA

Abstract Complex hybridity is a rare diploid genetic system of plants, extensively characterized in Oenothera, in which heterozygosity for one or more reciprocal translocations is maintained by means of autogamy and a balanced lethal system. It is visible at metaphase I of meiosis as rings or chains of chromosomes held together by terminalized chiasmata. Phylogenetic analysis based on 274 random amplified polymorphic DNA markers showed that in the Australian endemic Isotoma petraea (Lobeliaceae) the genetic system had a single origin, as a ring‐of‐six, in the Pigeon Rock population. It subsequently spread to other populations to produce hybrids incorporating additional chromosomes into the rings. Our research supports the suggestion that complex hybridity is an evolutionary response to intense inbreeding and selected because it allows masking of accumulated deleterious alleles.