About D. Sechellia
D. sechellia is endemic to the Seychelles Archipelago, a collection of coralline and granitic islands in the Indian ocean several hundred kilometers off the east coast of Africa. These islands are home to a number of endemic plants and animals. Permanent settlement of these islands by humans began about 400 years ago, although these islands may have been visited occasionally before then. With human settlement, a number of species were introduced. DNA evidence suggests, however, that D. sechellia inhabited these islands well before humans arrived (Kliman et al. 2000).
As first reported by Tsacas and Bächli (1981), D. sechellia is typically found near the fruit of the rubiaceous shrub, Morinda citrifolia. This small tree is common in the Seychelles, often inhabiting shorelines but also found at higher elevations (Sauer 1967; Robertson 1989). It has been cataloged on many of the islands in the Seychelles archipelago (Robertson 1989) and has also been found on Mauritius and Madagascar (Sauer 1961; Baker 1970). Morinda is also common throughout the Indian Ocean, Malaysia, and the islands of the Pacific. When Morinda arrived in the Seychelles is not known. It is most likely that Morinda fruit – which can survive salt water for more than a year – floated to the shore islands some time in the ancient past (Sauer 1967).
When and how D. sechellia arrived in the Seychelles is not known either. Presumably, a D. simulans-like ancestor was blown from the coast of Africa (or Madagascar) and settled on an island of the Seychelles Archipelago. From here, it colonized several other islands of the Seychelles. (D. sechellia has been collected on Praslin, Cousin, Frigate and Mahé islands.)
After arriving in the Seychelles, D. sechellia shifted from being a D. simulans-like generalist, to specializing on the fruit of M. citrifolia. Morinda fruit is toxic to many insects (Legal and Plawecki 1995). Why D. sechellia specialized on this normally toxic plant is not clear. Morinda fruit is abundant year round and maybe the only readily available host on the smaller islands of the Seychelles – although the main island, Mahé, surely provides a variety of other hosts. Alternatively, D. sechellia may have been driven to use Morinda by interspecific competition from other fruit flies such as D. malerkotliana or D. simulans, which are sympatric with D. sechellia (Louis and David 1986; R’Kha et al. 1997). Another possibility is that D. sechellia may have moved to a toxic host to avoid predation by parasitoid wasps such as Leptopilina species which are also found on the Seychelles (Louis and David 1986). At this point, simply not enough is known to suggest which is the more plausible scenario.
To use Morinda fruit as its host, D. sechellia evolved resistance to the toxins in this fruit. R’Kha et al. (1991) showed that media containing Morinda fruit pulp was toxic to D. simulans, D. mauritiana, D. melanogaster, D. ananassae, and D. malerkotliana, but not to D. sechellia. Legal et al. (1994) showed that octanoic acid, which constitutes 58% of the identifiable volatile compounds in ripe Morinda fruit (hexanoic acid, a closely related compound, comes in a distant second at 19%), is the primary source of the toxicity of the fruit (Legal et al. 1994 Farine et al 1996). They also showed that D. sechellia is highly resistant to the toxic effects of octanoic acid.
As Morinda fruit rots levels of octanoic acid decline. Interestingly, D. sechellia shows much less resistance to the volatiles that become common in rotten fruit (Legal et al.1994). In fact, D. sechellia is less tolerant of ethanol than its close relatives (Mercot et al. 1994). This result is intriguing as most Drosophila are saprophagous – that is, they feed on decaying, partially fermented resources. D. sechellia, on the other hand, appears to be better adapted at using the relatively unspoiled ripe Morinda.
Field and laboratory studies have shown that D. sechellia is strongly attracted to ripe Morinda and that this attraction is primarily mediated through octanoic acid, although other volatile compounds play a role as well (Louis and David 1986; R’Kha et al. 1991, Higa and Fuyama 1993; Amlou et al. 1998b; Legal et al. 1999). Relatively low concentrations of octanoic acid (0.1% by weight) have been shown to repulse D. simulans, D. mauritiana, and D. melanogaster, yet attract D. sechellia (Figure 1) (Amlou et al. 1998b; Legal et al. 1999). Hexanoic acid also has this effect, but only when higher concentrations are used, which is surprising given greater vapor pressure of hexanoic acid (Amlou et al. 1998b). These data and the fact that octanoic acid is three times more abundant in Morinda fruit than hexanoic acid suggest that octanoic acid is the primary attractant in nature.
The host preference behavior of D. sechellia involves chemotaxis, oviposition site preference, and stimulation of egg production. Louis and David (1986) demonstrated that D. sechellia is attracted to Morinda fruit in the field and the lab. In a set of release and recapture experiments, R’Kha et al. (1991) showed that D. sechellia, unlike D. simulans, will find and choose Morinda fruit over a banana bait 98% of the time, even when released 150m away. Legal et al. (1999) suggested that part of this attraction is likely due to octanoic and hexanoic acid.
R’Kha et al. (1991) also showed that D. sechellia exhibited strong attraction oviposition site preference for media containing Morinda fruit. Subsequently, it has been shown that D. sechellia’s oviposition site preference is strongly influenced by octanoic and hexanoic acid (Higa and Fuyama 1993; Amlou et al. 1998b; Legal et al. 1999). D. simulans and D. melanogaster both avoid laying eggs on media containing either of these acids. Interestingly, ethyl esters of these acids, which are common components of rotting fruit, do not cause the same species specific behaviors (Legal et al. 1999).
The presence of Morinda also appears to stimulate egg production in D. sechellia (R’Kha et al. 1991). In general, D. sechellia shows a 5-10 fold lower rate of egg production than its sister species (Coyne et al. 1991; R’Kha et al. 1997). This effect is partially explained by the fact that D. sechellia has only 50%-60% as many ovarioles as its sister species. Additionally, when not allowed to oviposit on Morinda, the number of eggs produced by each ovariole in D. sechellia females is about 60% that of its close relatives. When allowed to oviposit on Morinda, however, the rate of egg production per ovariole increases, again suggesting the D. sechellia prefers to use Morinda fruit as its host.
Species Relationships
D. sechellia is a member of the D. melanogaster subgroup and is most closely related to D. simulans and D. mauritiana. Which of these two species is the closer relative is not known, although recent evidence tentatively suggests that D. sechellia speciated before the split between D. simulans and D. mauritiana (Kliman et al 2000). The genetics of reproductive isolation in this group has been recently reviewed by Coyne and Orr (1998; see related Macdonald and Goldstein 1999). Thus, I will only discuss the basic biology of interspecific hybrids relevant to conducting genetic analyses of D. sechellia.
Both D. simulans and D. mauritiana produce fertile females and sterile males when crossed to D. sechellia regardless of the direction of the cross. (Wolbachia bacteria, while present in some strains of all three species do not appear to greatly affect the fertility or viability of hybrids [Giordano et al 1995].) This means that backcross hybrids can be generated between these species. This allows us to take advantage of the genetic tools available in these species including a number of genetic markers, a few chromosomal abberations, and some marker P-element insertion lines (Flybase 1999; True et al. 1996). It has also been shown that transgenic flies can be made in these species (True et al. 1996; Scavarda and Hartl 1984)
Typical for the D. simulans clade, D. melanogaster females when crossed to D. sechellia males produce only sterile F1 daughters, whereas D. melanogaster males when crossed to D. sechellia females produce only sterile F1 sons. A number of hybrid rescue mutations have been discovered in D. melanogaster and D. simulans (Ashburner 1989). These mutations typically lead to the production of both sterile males and females. Some combinations of these mutations can weakly restore the fertility of hybrids (Davis et al., 1996; Barbash et al. 2003). Sadly, D. sechellia seems to be more recalcitrant to hybrid rescue that its sister species (Barbash et al. 2000; Barbash et al. 2003). This means that only those D. melanogaster genetic tools that are informative in F1 hybrids (e.g. deficiencies) are useful.
Genetics in D. sechellia
Relative to D. melanogaster (or even D. simulans) the genetic tools available in D. sechellia are sparse. Several visible genetic markers are available and, recently, a number of molecular markers have been developed (Flybase 1999; Rux and Coyne 1991; Colson et al. 1999). However, most mapping studies using visible markers have taken advantage of the far more plentiful tools available in D. simulans via interspecific hybrids. Unfortunately, these studies are still of limited resolution and power.
In principle, it is possible to use many of the chromosomal deficiencies and duplications available in D. melanogaster to map traits in F1 hybrids between it and D. sechellia. In practice, however, this mapping approach is frustrated by three facts. (1) The viability of F1 hybrids between D. melanogaster and D. sechellia is poor and gets worse in hybrids with a chromosomal aberration (Barbash et al. 2000; Jones unpublished). (2) F1 melanogaster/sechellia hybrids show a number of morphological abnormalities including degenerated reproductive organs, bristle loss, malformed cuticle, and other morphological defects (Takano 1998). (3) D. sechellia is not completely chromosomally homosequential with D. melanogaster, which means a few regions cannot be adequately analyzed using deficiencies (Lemeunier and Ashburner 1984).
Recently, Coulson et al. (1999) expanded the number of genetic tools available in D. sechellia by developing a set of microsatellite markers that distinguish D. sechellia from D. simulans. The future development of molecular markers like these has been greatly facilitated by the D. melanogaster genome project – and will soon be further simplified by the genomic sequencing of D. simulans (D.J. Begun, pers. comm.).
Links
Seychelles
Nature Protection Trust of the Seychelles
Island Conservation Society
