This is the oldest English language journal in genetics. Founded by W. Bateson and R. C. Punnett in 1910, the Journal of Genetics was later edited by J. B. S. Haldane; later, Haldane and his wife, Helen Spurway brought the journal with them to India. After his death in 1964, Mrs. Haldane continued publication of the journal, with Madhav Gadgil and H. Sharat Chandra (Indian Institute of Science, Bangalore) and Suresh Jayakar (Laboratorio de Genetica Biochimica ed Evoluzionistica, Pavia) as editors, until her own death in 1977. In 1985, with the permission of Haldane’s heirs, the Indian Academy of Sciences resumed publication of the journal. The journal retains its traditional interest in evolutionary research that is of relevance to geneticists, even if not explicitly genetic in nature.

The journal of genetics

Sex chromosomes evolved from autosomes many times across the eukaryote phylogeny. Several models have been proposed to explain this transition, some involving male and female sterility mutations linked in a region of suppressed recombination between X and Y (or Z/W, U/V) chromosomes. Comparative and experimental analysis of a reference genome assembly for a double haploid YY male garden asparagus (Asparagus officinalis L.) individual implicates separate but linked genes as responsible for sex determination. Dioecy has evolved recently within Asparagus and sex chromosomes are cytogenetically identical with the Y, harboring a megabase segment that is missing from the X. We show that deletion of this entire region results in a male-to-female conversion, whereas loss of a single suppressor of female development drives male-to-hermaphrodite conversion. A single copy anther-specific gene with a male sterile Arabidopsis knockout phenotype is also in the Y-specific region, supporting a two-gene model for sex chromosome evolution.

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The asparagus genome sheds light on the origin and evolution of a young y chromosome

Dioecy, the presence of male and female flowers on separate individuals, is both widespread and uncommon within flowering plants, with only a few percent of dioecious species spread across most major phylogenetic taxa. It is therefore safe to assume that dioecy evolved independently in these different groups, which allows us to ask questions regarding the molecular and developmental mechanisms underlying these independent transitions to dioecy. We start this review by examining the problem from the standpoint of a genetic engineer trying to develop dioecy, discuss various potential solutions, and compare them to models proposed in the past and based on genetic and evolutionary considerations. Next, we present recent information regarding candidate sex determinants in three species, acquired using newly established genomic approaches. Although such specific information is still scarce, it is slowly becoming apparent that various genes or pathways can be altered to evolve dioecy.

One Hundred Ways to Invent the Sexes: Theoretical and Observed Paths to Dioecy in Plants.

Sex chromosomes have evolved independently in phylogenetically diverse flowering plant lineages. The genes governing sex determination in dioecious species remain unknown, but theory predicts that the linkage of genes influencing male and female function will spur the origin and early evolution of sex chromosomes. For example, in an XY system, the origin of an active Y may be spurred by the linkage of female suppressing and male promoting genes. Garden asparagus (Asparagus officinalis) serves as a model for plant sex chromosome evolution, given that it has recently evolved an XX/XY sex chromosome system. In order to elucidate the molecular basis of gender differences and sex determination, we used RNA-sequencing (RNA-Seq) to identify differentially expressed genes between female (XX), male (XY) and supermale (YY) individuals. We identified 570 differentially expressed genes, and showed that significantly more genes exhibited male-biased than female-biased expression in garden asparagus. In the context of anther development, we identified genes involved in pollen microspore and tapetum development that were specifically expressed in males and supermales. Comparative analysis of genes in the Arabidopsis thaliana, Zea mays and Oryza sativa anther development pathways shows that anther sterility in females probably occurs through interruption of tapetum development before microspore meiosis.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/nph.13389

Sex-biased gene expression in dioecious garden asparagus (Asparagus officinalis).

Hundreds of land plant lineages have independently evolved separate sexes in either gametophytes (dioicy) or sporophytes (dioecy), but 43% of all dioecious angiosperms are found in just 34 entirely dioecious clades, suggesting that their mode of sex determination evolved a long time ago. Here, we review recent insights on the molecular mechanisms that underlie the evolutionary change from individuals that each produce male and female gametes to individuals specializing in the production of just one type of gamete. The canonical model of sex chromosome evolution in plants predicts that two sex-determining genes will become linked in a sex-determining region (SDR), followed by expanding recombination suppression, chromosome differentiation and, ultimately, degeneration. Experimental work, however, is showing that single genes function as master regulators in model systems, such as the liverwort Marchantia and the angiosperms Diospyros and Populus. In Populus, this type of regulatory function has been demonstrated by genome editing. In other systems, including Actinidia, Asparagus and Vitis, two coinherited factors appear to independently regulate female and male function, yet sex chromosome differentiation has remained low. We discuss the best-understood systems and evolutionary pathways to dioecy, and present a meta-analysis of the sizes and ages of SDRs. We propose that limited sexual conflict explains why most SDRs are small and sex chromosomes remain homomorphic. It appears that models of increasing recombination suppression with age do not apply because selection favours mechanisms in which sex determination depends on minimal differences, keeping it surgically precise.

Plant sex chromosomes defy evolutionary models of expanding recombination suppression and genetic degeneration

Garden asparagus (Asparagus officinalis L.) is an economically important plant with 2n = 2x = 20 chromosomes and a haploid genome size of 1323 Mb (Bennett and Leitch 2003). The karyotype of asparagus consists of five long (L), one medium (M) and four small (S) chromosomes (Löptien 1976). Plants of this dioecious species differ in producing either male or female flowers. The sexual dimorphism in asparagus is controlled by a region located on a pair of homomorphic sex chromosomes (chromosome L5) (Löptien 1979) termed the M locus (Flory 1932; Uno et al. 2002). Maleness is dominant with males normally heterogametic (Mm) and females homogametic (mm). Supermales (MM) can be produced from anther culture or from rarely occurring andromonoecious plants. The identification of chromosome L5 is important for studying sex chromosome evolution in asparagus. However, it was observed that the sex chromosome pair is homomorphic: X and Y chromosomes do not differ in morphology. Telgmann-Rauber et al. (2007) tried to clone the region determining sex in asparagus from its position in the genome. Molecular cytogenetic and sequence analyses of bacterial artificial chromosomes (BACs) flanking M locus indicate that the BAC contain highly repetitive sequences that localize to centromeric and pericentromeric locations on all asparagus chromosomes. However, the L5 chromosome could not be distinguished by BAC-FISH (Telgmann-Rauber et al. 2007). Recently, FISH has become a powerful and useful tool for the direct detection of specific DNA in genome. In this technique, ribosomal DNA genes (45S and 5S rDNA) are commonly used as markers for the physical mapping of plant chromosomes to analyse genomic organization (Nakayama et al. 2001; Lan et al. 2006; Grabowska-Joachimiak et al.

Karyotype of asparagus by physical mapping of 45S and 5S rDNA by FISH.

To identify rapidly a number of genes probably involved in sex determination and differentiation of the dioecious plant Asparagus officinalis, gene expression profiles in early flower development for male and female plants were investigated by microarray assay with 8,665 probes. In total, 638 male-biased and 543 female-biased genes were identified. These genes with biased-expression for male and female were involved in a variety of processes associated with molecular functions, cellular components, and biological processes, suggesting that a complex mechanism underlies the sex development of asparagus. Among the differentially expressed genes involved in the reproductive process, a number of genes associated with floral development were identified. Reverse transcription-PCR was performed for validation, and the results were largely consistent with those obtained by microarray analysis. The findings of this study might contribute to understanding of the molecular mechanisms of sex determination and differe...

https://www.tandfonline.com/doi/pdf/10.1271/bbb.120943

Comparative Analysis of Gene Expression by Microarray Analysis of Male and Female Flowers of Asparagus officinalis

Garden asparagus (Asparagus officinalis L.) is a dioecious species whose male and female flowers are found in separate unisexual individuals. A region called the M-locus, located on a pair of homomorphic sex chromosomes, controls sexual dimorphism in asparagus. To date, no sex determining gene has been isolated from asparagus. To identify more genes involved in flower development in asparagus, subtractive hybridization library of male flowers in asparagus was constructed by suppression subtraction hybridization. A total of 107 expressed sequence tags (ESTs) were identified. BLASTX analysis showed that the library contained several genes that could be related to flower development. The expression patterns of seven selected genes believed to be involved in the development of asparagus male flower were further analyzed by semi-quantitative or real-time reverse-transcription polymerase chain reaction (RT-PCR). Results showed that AOEST4-5, AOEST12-40, and AOEST13-38 were strongly expressed in the male flower stage, whereas no transcript level of AOEST13-38 was detected in the female flower stage. The expression levels of AOEST13-87, AOEST13-92, AOEST13-40, and AOEST18-87 in the male flower stage were also higher than those in the female flower stage, although these transcripts were also expressed in other tissues. The identified genes can provide a strong starting point for further studies on the underlying molecular differences between the male and female flowers of asparagus.

Ning-Na Wang

Papers

Karyotype of asparagus by physical mapping of 45S and 5S rDNA by FISH.

Comparative Analysis of Gene Expression by Microarray Analysis of Male and Female Flowers of Asparagus officinalis

Isolation of differentially expressed sex genes in garden asparagus using suppression subtractive hybridization.