Detection of genetic incompatibilities in non-model systems using simple genetic markers: hybrid breakdown in the haplodiploid spider mite Tetranychus evansi.

When two related species interbreed, their hybrid offspring frequently suffer from reduced fitness. The genetics of hybrid incompatibility are described by the Bateson-Dobzhansky-Muller (BDM) model, where fitness is reduced by epistatic interactions between alleles of heterospecific origin. Unfortunately, most empirical evidence for the BDM model comes from a few well-studied model organisms, restricting our genetic understanding of hybrid incompatibilities to limited taxa. These systems are predominantly diploid and incompatibility is often complete, which complicates the detection of recessive allelic interactions and excludes the possibility to study viable or intermediate stages. Here, we advocate research into non-model organisms with haploid or haplodiploid reproductive systems and incomplete hybrid incompatibility because (1) dominance is absent in haploids and (2) incomplete incompatibility allows comparing affected with unaffected individuals. We describe a novel two-locus statistic specifying the frequency of individuals for which two alleles co-occur. This approach to studying BDM incompatibilities requires genotypic characterization of hybrid individuals, but not genetic mapping or genome sequencing. To illustrate our approach, we investigated genetic causes for hybrid incompatibility between differentiated lineages of the haplodiploid spider mite Tetranychus evansi, and show that strong, but incomplete, hybrid breakdown occurs. In addition, by comparing the genotypes of viable hybrid males and inviable hybrid male eggs for eight microsatellite loci, we show that nuclear and cytonuclear BDM interactions constitute the basis of hybrid incompatibility in this species. Our approach opens up possibilities to study BDM interactions in non-model taxa, and may give further insight into the genetic mechanisms behind hybrid incompatibility.

Impairment of the visuo-spatial sketch pad.

A case study is presented of a female patient, ELD, who has difficulty in the immediate recall of short sequences of visuo-spatial material following a right-hemisphere aneurysm. Despite poor performance on tasks such as the Brooks Matrix and the Corsi Blocks, ELD is good at the immediate serial recall of letters even when presentation modality is visual and shows effects of phonological similarity and articulatory suppression. This pattern of performance represents a double dissociation from that which has been observed with the short-term memory patient PV (Vallar & Baddeley, 1984), who is extremely poor at serial recall of verbal material but shows no visual memory impairment. It is argued that ELD has an impairment to the visuo-spatial component of working memory (Baddeley, 1986) in the absence of any phonological loop deficit. Further investigation reveals that ELD performs poorly on mental rotation tasks and finds it difficult to use imagery mnemonics, but has no difficulty in retrieving visuo-spatial information from long-term memory so long as it was learnt before her illness.

Impaired memory for new visual forms.

A case study is presented of a female patient who has had impairment of visual recent memory following damage to the right cerebral hemisphere from rupture of a middle cerebral artery aneurysm. Her ability to remember unfamiliar faces is severely impaired in comparison with controls, as is her ability to identify the faces of celebrities who have become famous since the time of her illness in 1985. By contrast, she performs well on tests of recognition memory for words and has no problem in identifying celebrities from their names. She is also good at identifying faces of people who became famous before her illness. On tests of episodic memory, she continues to perform well at remembering familiar faces and objects, regardless of whether the test demands memory for which face/object was presented or memory for which view of a face/object was presented. Her poor memory for visual material is confined to unfamiliar faces and objects. Consequently, it is evident that she can learn new visual information so long as it relates to a familiar visual form; it is the learning of new visual forms that is impaired. A final experiment revealed that she is also poor at remembering unfamiliar voices. This raises the possibility that the basic deficit affects recent nonverbal memory and is not confined to the visual modality. Overall, the findings have important implications for the nature of recent visual memory loss, and for the way in which visual information is stored in memory.

Aggressive behaviour of children aged 6-11: gender differences and their magnitude.

A total of 72 boys and 72 girls, of six age groups from 6 to 11 years, was observed in the school classroom over eight 5-minute periods, under conditions of low structure, non-directive teaching; aggressive and other categories of behaviour were recorded. There were no overall gender differences in aggression, but boys showed significantly more physical aggression than girls (overall effect size d = 0.36), whereas girls showed significantly more verbal aggression than boys (overall effect size d = 0.66). The physical aggression difference was variable both in direction and magnitude across the age categories and included a large reversal at 9 years. Boy's higher levels of physical aggression were not associated with higher levels of activity or more time spent in social interactions. The gender difference in verbal aggression was consistent in direction and magnitude across age categories, but in the opposite direction to the majority of previous findings. There were no gender differences in other categories of verbal behaviour.