Chromosome-level genome of putative autohexaploid Actinidia deliciosa provides insights into polyploidisation and evolution

Actinidia ('Mihoutao' in Chinese) includes species with complex ploidy, among which diploid Actinidia chinensis and hexaploid Actinidia deliciosa are economically and nutritionally important fruit crops. Actinidia deliciosa has been proposed to be an autohexaploid (2n = 174) with diploid A. chinensis (2n = 58) as the putative parent. A CCS-based assembly anchored to a high-resolution linkage map provided a chromosome-resolved genome for hexaploid A. deliciosa yielded a 3.91-Gb assembly of 174 pseudochromosomes comprising 29 homologous groups with 6 members each, which contain 39 854 genes with an average of 4.57 alleles per gene. Here we provide evidence that much of the hexaploid genome matches diploid A. chinensis; 95.5% of homologous gene pairs exhibited >90% similarity. However, intragenome and intergenome comparisons of synteny indicate chromosomal changes. Our data, therefore, indicate that if A. deliciosa is an autoploid, chromosomal rearrangement occurred following autohexaploidy. A highly diversified pattern of gene expression and a history of rapid population expansion after polyploidisation likely facilitated the adaptation and niche differentiation of A. deliciosa in nature. The allele-defined hexaploid genome of A. deliciosa provides new genomic resources to accelerate crop improvement and to understand polyploid genome evolution.

Characterisation of a Spontaneous Mutant of Lemna gibba G3 (Lemnaceae)

A spontaneous mutant of the duckweed Lemna gibba clone no. 7796 (known as strain G3, WT) was discovered. In this mutant clone, L. gibba clone no. 9602 (mt), the morphological parameters (frond length, frond width, root length, root diameter) indicated an enlarged size. A change in the frond shape was indicated by the decreased frond length/width ratio, which could have taxonomic consequences. Several different cell types in both the frond and the root were also enlarged. Flow cytometric measurements disclosed the genome size of the WT as 557 Mbp/1C and that of the mt strain as 1153 Mbp/1C. This represents the results of polyploidisation of a diploid clone to a tetraploid one. The mutant clone flowered under the influence of long day-treatment in half-strength Hutner's medium in striking contrast to the diploid WT. Low concentration of salicylic acid (<1 µM) induced flowering in the tetraploid mutant but not in the diploid plants. The transcript levels of nuclear-encoded genes of the photosynthetic apparatus (CAB, RBCS) showed higher abundance in light and less dramatic decline in darkness in the mt than in WT, while this was not the case with plastid-encoded genes (RBCL, PSAA, PSBA, PSBC).

Disentangling Relationships among the Alpine Species of Luzula Sect. Luzula (Juncaceae) in the Eastern Alps

Polyploidisation, agmatoploidy and symploidy have driven the diversification of Luzula sect. Luzula. Several morphologically very similar species with different karyotypes have evolved, but their evolutionary origins and relationships are unknown. In this study, we used a combination of relative genome size and karyotype estimations as well amplified fragment length polymorphism (AFLP) fingerprinting to investigate the relationships among predominately (sub)alpine Luzula alpina, L. exspectata, L multiflora and L. sudetica in the Eastern Alps, including also some samples of L. campestris and L. taurica as outgroup. Our study revealed common co-occurrence of two or three different ploidies (di-, tetra- and hexaploids) at the same localities, and thus also common co-occurrence of different species, of which L. sudetica was morphologically, ecologically and genetically most divergent. Whereas agmatoploid L. exspectata likely originated only once from the Balkan L. taurica, and hexaploid L. multiflora once from tetraploid L. multiflora, the AFLP data suggest multiple origins of tetraploid L. multiflora, from which partly agmatoploid individuals of L. alpina likely originated recurrently by partial fragmentation of the chromosomes. In contrast to common recurrent formation of polyploids in flowering plants, populations of agmatoploids resulting by fission of complete chromosome sets appear to have single origins, whereas partial agmatoploids are formed recurrently. Whether this is a general pattern in Luzula sect. Luzula, and whether segregation of ecological niches supports the frequent co-occurrence of closely related cytotypes in mixed populations, remains the subject of ongoing research.

Invasion and genome reproduction of the trophoblast cells of placenta junctional zone in the field vole, Microtus rossiaemeridionalis

In the field vole Microtus rossiaemeridionalis, like in other rodents, invasive secondary giant trophoblast cells (SGTC) form a continuous layer at the foeto-maternal interface in the beginning of placentation. However, in the field vole, at midgestation, clusters of junctional zone (JZ) trophoblast non-giant cells interrupt SGTC layer and progressively replace SGTC at the border of decidua basalis. As a result, 'border' cells form a continuous stratum of cytokeratin-positive glycogen-rich cells at the foeto-maternal interface. SGTC plunge into JZ and line the lacunae with maternal blood. SGTC are bound by their highly cytokeratin-positive sprouts forming a framework that holds other trophoblast cell populations. According to DNA cytophotometry, the 'border' cells show the highest ploidy among the JZ cells (up to 46% of 8c cells). Thus, in M. rossiaemeridionalis the role of barrier between semiallogenic foetal and maternal tissues is shifted from the highly endopolyploid (32c-1024c) SGTC to the specific subpopulation of glycogen-rich non-giant (2c-16c) 'border' trophoblast cells that, however, exceed the ploidy of the deeply located and/or proliferative JZ trophoblast cells.

Life and death among plant lysophosphatidic acid acyltransferases

The tetraploid Brassica napus possesses several seed-expressed microsomal lysophosphatidic acid acyltransferases (LPAAT ) including BAT1.5, which has been retained after genome duplication as a consequence of a subfunctionalisation of the gene encoding the ubiquitously expressed Kennedy pathway enzyme BAT1.13. Next, cDNA BAT1.3, encoding a LPAAT was subsequently isolated from an embryo library. The rapeseed LPAAT encoded by BAT1.3 is orthologous to the Arabidopsis thaliana At1g51260 gene product possibly associated with tapetum development and male fertility. However, BAT1.3 expression is predominant during the mid stages of embryo development in seeds of Brassica napus. Functional characterisation of BAT1.3 provides further support for a hypothesis of gene dosage sensitivity of LPAATs as does an analysis of the chromosomal localisation of LPAAT genes in Arabidopsis thaliana. The pattern of retention or loss of LPAAT genes after polyploidisation or segmental duplication is consistent with a model of balanced gene drive.