Litchi chinensis is a subtropical fruit crop, popular for its nutritional value and taste. Fruits with small seed size and thick aril are desirable in litchi. To gain molecular insight into gene expression that leads to the reduction in the size of seed in Litchi chinensis, transcriptomes of two genetically closely related genotypes, with contrasting seed size were compared in developing ovules. The cDNA library constructed from early developmental stages of ovules (0, 6, and 14 days after anthesis) of bold- and small-seeded litchi genotypes yielded 303,778,968 high quality paired-end reads. These were de-novo assembled into 1,19,939 transcripts with an average length of 865 bp. A total of 10,186 transcripts with contrast in expression were identified in developing ovules between the small- and large- seeded genotypes. A majority of these differences were present in ovules before anthesis, thus suggesting the role of maternal factors in seed development. A number of transcripts indicative of metabolic stress, expressed at higher level in the small seeded genotype. Several differentially expressed transcripts identified in such ovules showed homology with Arabidopsis genes associated with different stages of ovule development and embryogenesis.

BackgroundAnnona squamosa L., a popular fruit tree, is the most widely cultivated species of the genus Annona. The lack of transcriptomic and genomic information limits the scope of genome investigations in this important shrub. It bears aggregate fruits with numerous seeds. A few rare accessions with very few seeds have been reported for Annona. A massive pyrosequencing (Roche, 454 GS FLX+) of transcriptome from early stages of fruit development (0, 4, 8 and 12 days after pollination) was performed to produce expression datasets in two genotypes, Sitaphal and NMK-1, that show a contrast in the number of seeds set in fruits. The data reported here is the first source of genome-wide differential transcriptome sequence in two genotypes of A. squamosa, and identifies several candidate genes related to seed development.ResultsApproximately 1.9 million high-quality clean reads were obtained in the cDNA library from the developing fruits of both the genotypes, with an average length of about 568 bp. Quality-reads were assembled de novo into 2074 to 11004 contigs in the developing fruit samples at different stages of development. The contig sequence data of all the four stages of each genotype were combined into larger units resulting into 14921 (Sitaphal) and 14178 (NMK-1) unigenes, with a mean size of more than 1 Kb. Assembled unigenes were functionally annotated by querying against the protein sequences of five different public databases (NCBI non redundant, Prunus persica, Vitis vinifera, Fragaria vesca, and Amborella trichopoda), with an E-value cut-off of 10−5. A total of 4588 (Sitaphal) and 2502 (NMK-1) unigenes did not match any known protein in the NR database. These sequences could be genes specific to Annona sp. or belong to untranslated regions. Several of the unigenes representing pathways related to primary and secondary metabolism, and seed and fruit development expressed at a higher level in Sitaphal, the densely seeded cultivar in comparison to the poorly seeded NMK-1. A total of 2629 (Sitaphal) and 3445 (NMK-1) Simple Sequence Repeat (SSR) motifs were identified respectively in the two genotypes. These could be potential candidates for transcript based microsatellite analysis in A. squamosa.ConclusionThe present work provides early-stage fruit specific transcriptome sequence resource for A. squamosa. This repository will serve as a useful resource for investigating the molecular mechanisms of fruit development, and improvement of fruit related traits in A. squamosa and related species.Electronic supplementary materialThe online version of this article (doi:10.1186/s12864-015-1248-3) contains supplementary material, which is available to authorized users.

Amplified fragment length polymorphism (AFLP) was used to assess genetic relatedness among 23 popular commercial cultivars of lychee (Litchi chinensis) and identify the markers associated with small-seeded cultivars. A combination of three primer pairs with high discriminatory ability was selected for resolving the identity of lychee cultivars. The marker system resolved the instances of synonymies and homonymies. The two cultivars, Dehrarose and Dehradun, whose names are often used interchangeably, could be discriminated by AFLP. A cultivar, Calcuttia (H), grown in the Punjab region was found genetically very similar to China (M) of Bihar region in India, suggesting that both could be the same clone. A number of cultivars with contrasting seed size were included in the study. The cultivars Bedana, Seedless Late, and China were positioned in one group, unlike the grouping previously reported based on morphological traits. The AFLP fingerprinting revealed five marker fragments exclusive to the genotypes, which bear fruit with small seed and large pulp.

No abstract

Litchi is a sub-tropical fruit crop with contrasting genotypes bearing fruits with variable seed size. Small seed size is a desirable trait in litchi, as it improves consumer preference and facilitates fruit processing. Seed speci c transcriptome analysis was performed in two litchi genotypes with contrasting seed size to identify the genes associated with seed development in litchi fruits. The transcriptomic data from seeds at mid-development stages (16 to 28 days after anthesis) were de-novo assembled into 1,39,608 Trinity transcripts. Out of these, 6,325 transcripts expressed differentially between the two contrasting genotypes. Several putative genes for salicylic acid, jasmonic acid and brassinosteriod pathways were down-regulated in the small-seeded litchi. The putative regulators of seed maturation and seed storage were down-regulated in the small-seeded genotype. Embryogenesis, cell expansion, seed size and stress related Trinity transcripts exhibited differential expression. Further studies can lead to identi cation and characterization of early regulators of seed size in litchi.