The objective of this study is to develop InDel molecular markers for Island cotton, which is characterized by its superior fiber quality, particularly the fiber tensile strength-a key indicator of cotton fiber quality. The study aims to validate these markers using RIL (Recombinant Inbred Line) populations and resource materials, thereby providing a theoretical foundation for breeding new varieties of Island cotton with enhanced fiber quality.

Utilizing a previously established population of 213 Pima S-7 and 5917 F_5:6 RILs, we conducted QTL (Quantitative Trait Locus) mapping to identify the locus regulating fiber strength in Island cotton, designated qFS-chr17-1. InDel markers were designed based on whole genome sequencing (WGS) data of the parental lines, followed by the identification of polymorphic markers. Preliminary validation of these markers was performed using 40 extreme family materials selected based on phenotypic data. Genotyping was carried out on both the 213 RIL population and the 213 Island cotton resource population, alongside multi-year fiber quality data to assess the markers' effectiveness.

The genotyping of the RIL and Island cotton resource populations with the two developed InDel markers indicated a close linkage to fiber strength phenotypic data, with significant differences observed in fiber strength traits among the differentiated materials. The analysis of genotypic combinations revealed an upward trend in fiber strength across four combination types, with materials exhibiting the Hap3 (B/A) and Hap4 (B/B) genotypes demonstrating significantly greater fiber strength than those with Hap1 (A/A) and Hap2 (A/B). Furthermore, the InDel-3L2 marker showed significant correlations with fiber length, fiber uniformity, and spinning consistency index, consistent with the observed phenotypic trends. Analysis of multi-year fiber quality data from two experimental sites revealed environmental variability in fiber quality, while temperature data indicated that the developed molecular markers are minimally influenced by environmental factors. Clustering analysis of fiber quality data from 213 Island cotton resource materials, combined with molecular marker genotyping, identified eight materials exhibiting superior fiber quality.

This study successfully developed two InDel molecular markers closely linked to the fiber strength QTL (qFS-chr17-1), which maintain their effectiveness upon combination. The InDel-3L2 marker demonstrates significant correlations with fiber length, fiber uniformity, and spinning consistency index. These markers can efficiently and accurately identify high-strength fiber resources in Island cotton, contributing to the breeding of improved fiber quality. Additionally, eight materials with excellent fiber quality have been identified.