The Hessian fly (Mayetiola destructor
) is an important pest of wheat (Triticum
spp.) and a member of one of the largest and most economically important families of insects, the gall midges (Cecidomyiidae). Its genetic tractability and short generation time (~28 days) make it especially attractive as an experimental model for investigating insect-plant interactions [1
]. This capacity was first demonstrated when it was the first insect shown to have a gene-for-gene interaction with its host plant [2
]. Previously, it was used to study chromosome elimination and the function of the germ-line-limited "E" chromosomes that characterize all gall midge species [3
]. Later investigations demonstrated that it has other biological attributes that are compatible with genomic analyses, including polytene chromosomes in the larval salivary glands [6
] and a small genome (158 Mb) [7
]. The development of a physically anchored genetic map [8
], a syntenic analysis of a BAC-based contig [9
], and transcriptomic analyses of the first instar salivary glands [10
] also demonstrated the potential of this insect for comparative genomics with other dipteran species.
Improved genomic maps are essential to further develop this insect as an experimental organism. However, certain biological characteristics make fine-scale genetic mapping in the Hessian fly problematic. The first problem is the relatively low number of offspring produced by single females, typically ranging from 50 to 200, which limits the resolution of a genetic map. The second problem is the small size (2 to 3 mm in length) of the insect. This severely limits the yield of the genomic DNA extracted from individuals, and the number of molecular markers that can be genetically mapped. A third problem, the insect's unusual chromosome cycle and mechanism of sex determination, makes the construction of inbred strains difficult. Because this phenomenon is both central to the genetics of the insect and worthy of genomic investigation, it is briefly described below.
The germ line of the Hessian fly contains both maternally and paternally derived copies of each of two autosomes (A1 and A2) and two X chromosomes (X1 and X2) [12
]. It also contains a variable number (~32) of maternally inherited germ-line-limited "E" chromosomes (A1A2X1X2/A1A2X1X2; E). The E chromosomes are eliminated from all future somatic cells during the fifth cleavage division of embryogenesis [4
]. This post-zygotic division also determines sex. Embryos that eliminate the paternally inherited X chromosomes, together with the E chromosomes, possess a male determining somatic karyotype (A1A2X1X2/A1A2OO) and develop as males. Those that retain both sets of X chromosomes possess a female determining somatic karyotype (A1A2X1X2/A1A2X1X2) and develop as females. Maternal genotype determines whether the paternally derived X chromosomes are eliminated or retained in the soma [13
]. As a consequence, an unusual mating structure is established that is resistant to inbreeding; females produce either all-female or all-male families. Chromosome elimination also occurs during spermatogenesis. The result is that all sperm carry only the maternally derived autosomes and X chromosomes (A1A2X1X2). Genetic recombination between the homologous autosomes and X chromosomes occurs only during meiosis of oogenesis. All ova carry a single copy of each autosome and each X chromosome and a full complement (~32) of E chromosomes (A1A2X1X2; E). There is no heterogametic sex since all sperm and all ova are genetically equivalent and sex is determined after fertilization.
In contrast to the problems that complicate genetic analyses, the Hessian fly polytene chromosomes make physical mapping relatively simple. Although they lack the banding resolution of Drosophila
polytene chromosomes, they still provide a relatively high mapping resolution when bacterial artificial chromosomes (BACs) are used as probes for fluorescence in situ
hybridization (FISH) [8
]. This facility suggested that the development of a dense physical map of the Hessian fly's polytene chromosomes should precede high-resolution genetic mapping. We therefore applied the high-information content fingerprinting (HICF) method developed by Luo et al. [14
] to construct a BAC contig map using three Hessian fly BAC libraries (Table ). The result was a first generation FPC-based physical map of the Hessian fly genome that is anchored to the polytene chromosomes. It is based on restriction fingerprints of 13,614 BAC clones. The largest 266 contigs were positioned on the chromosomes by fluorescence in situ
hybridization (FISH). These contigs contain 4,563 BAC clones, and cover ~60% of the genome. The BAC end sequences (BES) of the clones used in this analysis were determined to provide the capacity to develop sequenced tagged site (STS) markers evenly distributed over the genome. BAC-end sequence data were also analyzed to improve our understanding of Hessian fly genome organization.
BAC libraries fingerprinted for the Hessian fly physical map