Advances in DNA sequencing have made it easier to sequence and assemble plant genomes. Here, we extend an earlier study, and compare recent methods for long read sequencing and assembly. Updated Oxford Nanopore Technology software improved assemblies. Using more accurate sequences produced by repeated sequencing of the same molecule (Pacific Biosciences HiFi) resulted in less fragmented assembly of sequencing reads. Using data for increased genome coverage resulted in longer contigs, but reduced total assembly length and improved genome completeness. The original model species, Macadamia jansenii, was also compared with three other Macadamia species, as well as avocado (Persea americana) and jojoba (Simmondsia chinensis). In these angiosperms, increasing sequence data volumes caused a linear increase in contig size, decreased assembly length and further improved already high completeness. Differences in genome size and sequence complexity influenced the success of assembly. Advances in long read sequencing technology continue to improve plant genome sequencing and assembly. However, results were improved by greater genome coverage, with the amount needed to achieve a particular level of assembly being species dependent.

SUMMARY: Most flowering plants are hermaphrodites, but around 6% of species are dioecious, having separate male and female plants. Sex chromosomes and some sex‐specific genes have been reported in plants, but the genome sequences have not been compared. We now report the genome sequence of male and female jojoba (Simmondsia chinensis) plants, revealing a very large difference in the sex chromosomes. The male genome assembly was 832 Mb and the female 822 Mb. This was explained by the large size differences in the Y chromosome (37.6 Mb) compared with the X chromosome (26.9 Mb). Relative to the X chromosome, the Y chromosome had two large insertions each of more than 5 Mb containing more than 400 genes. Many of the genes in the chromosome‐specific regions were novel. These male‐specific regions included many flowering‐related and stress response genes. Smaller insertions found only in the X chromosome totalled 877 kb. The wide divergence of the sex chromosomes suggests a long period of adaptation to diverging sex‐specific roles. Male and female plants may have evolved to accommodate factors such as differing reproductive resource allocation requirements under the stress of the desert environment in which the plants are found. The sex‐determining regions accumulate genes beneficial to each sex. This has required the evolution of many more novel sex‐specific genes than has been reported for other organisms. This suggest that dioecious plants provide a novel source of genes for manipulation of reproductive performance and environmental adaptation in crops.

Plant plastid genomes are highly conserved with most flowering plants having the same complement of essential plastid genes. Here, we report the loss of five of the eleven NADH dehydrogenase subunit genes (ndh) in the plastid of a desert plant jojoba (Simmondsia chinensis). The plastid genome of jojoba was 156,496 bp with one large single copy region (LSC), a very small single copy region (SSC) and two expanded inverted repeats (IRA + IRB). The NADH dehydrogenase (NDH) complex is comprised of several protein subunits, encoded by the ndh genes of the plastome and the nucleus. The ndh genes are critical to the proper functioning of the photosynthetic electron transport chain and protection of plants from oxidative stress. Most plants are known to contain all eleven ndh genes. Plants with missing or defective ndh genes are often heterotrophs either due to their complete or holo- or myco- parasitic nature. Plants with a defective NDH complex, caused by the deletion/pseudogenisation of some or all the ndh genes, survive in milder climates suggesting the likely extinction of plant lineages lacking these genes under harsh climates. Interestingly, some autotrophic plants do exist without ndh gene/s and can cope with high or low light. This implies that these plants are protected from oxidative stress by mechanisms excluding ndh genes. Jojoba has evolved mechanisms to cope with a non-functioning NDH complex and survives in extreme desert conditions with abundant sunlight and limited water.

Jojoba (Simmondsia Chinensis) is a perennial stress tolerant desert shrub that has oil containing seeds and inhabits the Sonoran desert in the southwest of the United States and northwest Mexico. It has attracted a growing worldwide interest for multi-purpose uses. However, the most attractive characteristics of Jojoba are the richness of the oil content of the seed and the superior stress tolerance of the plant. Little has been done towards Jojoba genetic improvement. The exploration of jojoba genetic resources will define a molecular and biochemical fingerprint for jojoba and will aid sustainable crop commercialisation define. In this research, we aim to establish a reference genome database for Jojoba, which will help to facilitate crop improvement. Besides, the contribution to reveal the molecular background of its outstanding drought tolerance using transcriptional profiling during a water stress. RNA sequencing will be performed for samples collected under moderate and severe stress. The genetic database of jojoba will help to reveal the genetic mechanism of response and identify the genes responsible for the drought tolerant phenotype of this crop. Application of this knowledge will support the researchers, farmers, and the Jojoba industry.