**5. Duplication in the multigene family**

In general, duplicated genes undergo either (i) concerted evolution or (ii) birth-and-death evolution (Nei & Rooney, 2005). In the concerted evolution model, all members of a gene family evolve as a unit rather than independently. When a mutation occurs in a gene, mutation spreads to all other gene members by unequal crossover or gene conversion. As a result, all members of the gene family show identical sequence to each other. The evolution of rRNA multigene families in vertebrates is a classic example of concerted evolution. Analysis of MHC genes in mammals (Hughes & Nei, 1989; Nei et al., 1997; Nei & Hughes, 1992), other immune system related genes (Hughes & Nei, 1990; Ota & Nei, 1994) and disease-related genes (Zhang et al., 2000) show a quite different evolutionary pattern. The birth-and-death evolution model was proposed to explain differential duplication/independent diversification processes that result to subsequent loss or maintenance of genes in a multigene family. Thus, some duplicated genes are maintained in the genome for a long time while others are deleted or became pseudogenes through deleterious mutations. This model applies to rRNAs of *Plasmodium* species in marked contrast to the concerted evolution of rRNAs in most organisms. The model aptly explains the observation that rRNA genes in *Plasmodium* were structurally and functionally distinct (Rooney, 2004; Nishimoto et al., 2008).

The observed gene duplication and gene deletion found in the *Plasmodium* SERA genes are clearly in concordance with the birth-and-death model, although traits of gene conversion are detected in a few of Group IV SERA genes. The birth-and-death model has, likewise, been recently proposed for gene duplication/gene deletion of merozoite surface protein 7, an immune target parasite surface antigen gene (Garzón-Ospina et al., 2010). It, thus, seem

Clues to Evolution of the SERA Multigene Family in the Genus *Plasmodium* 327

Group I SERA gene, bearing the canonical cysteine, was shown to be transcribed in the oocyst stage and its gene product was a protease required for sporozoite egress from the oocyst (Aly & Matuschewski, 2005). Group III SERA gene was suggested to play an essential role during schizont rupture/merozoite release in the mammalian host (Yeoh et al., 2007; Arastu-Kapur et al., 2008; Putrianti et al., 2010). Group IV SERA genes bear the characteristic replacement of active-site cysteine to a serine residue. Perhaps importantly, only mammalian parasites have Group IV SERA gene; and Group IV SERA gene of primate parasite was suggested to play an essential role in schizont rupture/merozoite release together with Group III SERA gene (Yeoh et al., 2007; Arisue et al., 2011). The duplication of Group IV SERA gene occurred particularly frequent in two evolutionarily distinct primate lineages and it is intriguing to assume that duplications of SERA genes were associated with

The study of the SERA gene family points to its unique features reinforcing the importance of investigating other uncharacterized gene families of *Plasmodium* to further understand the evolutionary history and biology of this harmful parasite. Many questions still remain in the analysis of SERA. SERA genes are thought to be subject to birth-and-death evolution, and thus, a pattern of interspecific gene clustering is expected to characterize the SERA family whereby functional genes are maintained in the genome for a long time and others are deleted or become non-functional. Group I and Group III SERA genes are highly conserved in *Plasmodium* species. For Group II SERA genes, although maintained among *Plasmodium* species with significant sequence similarity, no function has yet been predicted. Gene disruption studies with Group II SERA gene of *P. berghei* showed no apparent phenotypic change (Arisue et al., unpublished data). Group II is similar to Group I and Group III in being a cysteine-type SERA gene which has been suggested to have proteolytic activity to cleave host membrane structure (Aly & Matuschewski, 2005; Yoeh et al., 2007). The papainlike cysteine protease motif in its amino acid sequence suggests the possibility that Group II SERA act as a protease sometime in the parasite life cycle. Parasite egress from the host cell

*P. falciparum* SERA5 is a vaccine candidate molecule now on clinical trial in Uganda (Horii et al., 2010). Serum antibodies against the N-terminal domain of *P. falciparum* SERA5 in individuals living in malaria endemic area protect infants from clinical malaria and inhibit *in vitro* parasite growth (Okech et al., 2001, 2006; Aoki et al., 2002; Horii et al., 2010). During

Fig. 10. The life cycle of the malaria parasite and inferred role(s) of SERA.

is an important process that remains poorly understood.

host range expansion.

most probable that diversification of *Plasmodium* SERA multigene family was also driven by the birth-and-death evolution. Inferred gene duplication events in the evolution of the *Plasmodium* SERA gene family are shown in Fig. 9.

Fig. 9. Inferred gene duplication events in the evolution of the *Plasmodium* SERA gene family. Asterisk in *P. reichenowi* denotes that the SERA gene number in this species is tentative.

*Theileria* is the only other genus to possess a SERA homolog gene. The apicomplexan parasite is closely related to *Plasmodium*. Sequence similarity search against *Theileria* genome database at The Sanger Institute identified a single gene from both *T. parva* and *T. annulata* which has similarity with cysteine-type SERA (Arisue et al., 2007; McCoubrie et al., 2007). Based on Fig. 9, as all *Plasmodium* species have multiple SERA genes, we likely infer that the first duplication occurred at the common ancestor lineage of *Plasmodium.* Because every *Plasmodium* species has Group I SERA gene, the first duplication event is from Group I SERA gene. The duplication events which gave rise to Group II and IV SERA genes occurred after the divergence of *P. gallinaceum* from the branch leading to a common ancestral species of other *Plasmodium* species since *P. gallinaceum* has no Group II to IV SERA gene. The rest of the 17 *Plasmodium* species might have diverged into five lineages of (i) *P. falciparum* and *P. reichenowi*, (ii) *P. ovale*, (iii) rodent *Plasmodium* species, (iv) *P. vivax* and *P. vivax*-related monkey parasite species, and (v) *P. malariae,* and duplications of Group IV SERA genes occurred independently on each lineage. In addition, gene deletions as well as pseudogenization/truncation occurred frequently in *P. vivax* and *P. vivax*-related primate parasite lineage.

### **6. Conclusion and open issues in SERA study**

Multigene families are believed to provide an organism with a set of related genes that allow fine tuning of its biological function with possibly different temporal or topologic expression patterns. SERA gene duplications during *Plasmodium* evolution generated four types of SERA genes: Group I to Group IV. The speculated function of SERA during the parasite life cycle is summarized in Fig. 10.

most probable that diversification of *Plasmodium* SERA multigene family was also driven by the birth-and-death evolution. Inferred gene duplication events in the evolution of the

Fig. 9. Inferred gene duplication events in the evolution of the *Plasmodium* SERA gene family. Asterisk in *P. reichenowi* denotes that the SERA gene number in this species is

*Theileria* is the only other genus to possess a SERA homolog gene. The apicomplexan parasite is closely related to *Plasmodium*. Sequence similarity search against *Theileria* genome database at The Sanger Institute identified a single gene from both *T. parva* and *T. annulata* which has similarity with cysteine-type SERA (Arisue et al., 2007; McCoubrie et al., 2007). Based on Fig. 9, as all *Plasmodium* species have multiple SERA genes, we likely infer that the first duplication occurred at the common ancestor lineage of *Plasmodium.* Because every *Plasmodium* species has Group I SERA gene, the first duplication event is from Group I SERA gene. The duplication events which gave rise to Group II and IV SERA genes occurred after the divergence of *P. gallinaceum* from the branch leading to a common ancestral species of other *Plasmodium* species since *P. gallinaceum* has no Group II to IV SERA gene. The rest of the 17 *Plasmodium* species might have diverged into five lineages of (i) *P. falciparum* and *P. reichenowi*, (ii) *P. ovale*, (iii) rodent *Plasmodium* species, (iv) *P. vivax* and *P. vivax*-related monkey parasite species, and (v) *P. malariae,* and duplications of Group IV SERA genes occurred independently on each lineage. In addition, gene deletions as well as pseudogenization/truncation occurred frequently in *P. vivax* and *P. vivax*-related primate

Multigene families are believed to provide an organism with a set of related genes that allow fine tuning of its biological function with possibly different temporal or topologic expression patterns. SERA gene duplications during *Plasmodium* evolution generated four types of SERA genes: Group I to Group IV. The speculated function of SERA during the

*Plasmodium* SERA gene family are shown in Fig. 9.

tentative.

parasite lineage.

**6. Conclusion and open issues in SERA study** 

parasite life cycle is summarized in Fig. 10.

Fig. 10. The life cycle of the malaria parasite and inferred role(s) of SERA.

Group I SERA gene, bearing the canonical cysteine, was shown to be transcribed in the oocyst stage and its gene product was a protease required for sporozoite egress from the oocyst (Aly & Matuschewski, 2005). Group III SERA gene was suggested to play an essential role during schizont rupture/merozoite release in the mammalian host (Yeoh et al., 2007; Arastu-Kapur et al., 2008; Putrianti et al., 2010). Group IV SERA genes bear the characteristic replacement of active-site cysteine to a serine residue. Perhaps importantly, only mammalian parasites have Group IV SERA gene; and Group IV SERA gene of primate parasite was suggested to play an essential role in schizont rupture/merozoite release together with Group III SERA gene (Yeoh et al., 2007; Arisue et al., 2011). The duplication of Group IV SERA gene occurred particularly frequent in two evolutionarily distinct primate lineages and it is intriguing to assume that duplications of SERA genes were associated with host range expansion.

The study of the SERA gene family points to its unique features reinforcing the importance of investigating other uncharacterized gene families of *Plasmodium* to further understand the evolutionary history and biology of this harmful parasite. Many questions still remain in the analysis of SERA. SERA genes are thought to be subject to birth-and-death evolution, and thus, a pattern of interspecific gene clustering is expected to characterize the SERA family whereby functional genes are maintained in the genome for a long time and others are deleted or become non-functional. Group I and Group III SERA genes are highly conserved in *Plasmodium* species. For Group II SERA genes, although maintained among *Plasmodium* species with significant sequence similarity, no function has yet been predicted. Gene disruption studies with Group II SERA gene of *P. berghei* showed no apparent phenotypic change (Arisue et al., unpublished data). Group II is similar to Group I and Group III in being a cysteine-type SERA gene which has been suggested to have proteolytic activity to cleave host membrane structure (Aly & Matuschewski, 2005; Yoeh et al., 2007). The papainlike cysteine protease motif in its amino acid sequence suggests the possibility that Group II SERA act as a protease sometime in the parasite life cycle. Parasite egress from the host cell is an important process that remains poorly understood.

*P. falciparum* SERA5 is a vaccine candidate molecule now on clinical trial in Uganda (Horii et al., 2010). Serum antibodies against the N-terminal domain of *P. falciparum* SERA5 in individuals living in malaria endemic area protect infants from clinical malaria and inhibit *in vitro* parasite growth (Okech et al., 2001, 2006; Aoki et al., 2002; Horii et al., 2010). During

Clues to Evolution of the SERA Multigene Family in the Genus *Plasmodium* 329

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blood stage growth, all SERA gene family member of *P. falciparum* are transcribed most actively at trophozoite and schizont stages. SERA5 is the most abundantly expressed gene family member, with expression levels estimated to be approximately 0.5-1.5% of the whole mRNA at schizont stage (Aoki et al, 2002). However, sero-positivity rate against the Nterminal domain of *P. falciparum* SERA5 was observed to be relatively low (Fig. 11.; Aoki et al., 2002; Horii et al., 2010).

Fig. 11. Relative sero-conversion rates of *P. falciparum* SERA3 to SERA6 and merozoite surface protein 1 (MSP1) in a malaria endemic area of Uganda.

Since the SERA gene family does not show antigenic variation to evade host immune response (Fig. 7.), there may possibly be another mechanism of host parasite evasion/molecular mimicry/interference or competition.

The host range or host specificity of *Plasmodium* is believed to be restricted, although, primate malaria parasites generally infect multiple hosts. For example, it has been reported that, *P. knowlesi* and *P. cynomolgi* have the ability to infect a wide variety of macaques and human (Coatney et al., 1971); additionally, two human parasites *P. malariae* and *P. ovale* have been detected in chimpanzees (Hayakawa et al., 2009; Duval et al., 2009). It may be probable that duplications of Group IV SERA genes that occurred frequently in both primate parasite lineages may be associated with host range expansions. To date, no experimental support lends credence to this speculation.

As described above, the molecular function of SERA genes in each group, the relationship of immune evasion mechanism and the SERA gene family, and the association of host range with Group IV SERA genes remain important issues that needs to be addressed. The importance of SERA genes in parasite egress and their role in host-parasite interactions serve to propel further studies in understanding this multigene family.

### **7. Acknowledgment**

This work was supported by **KAKENHI (18073013 and 20390120)** from the Japanese Ministry of Education, Science, Sports, Culture and Technology.

## **8. References**

328 Gene Duplication

blood stage growth, all SERA gene family member of *P. falciparum* are transcribed most actively at trophozoite and schizont stages. SERA5 is the most abundantly expressed gene family member, with expression levels estimated to be approximately 0.5-1.5% of the whole mRNA at schizont stage (Aoki et al, 2002). However, sero-positivity rate against the Nterminal domain of *P. falciparum* SERA5 was observed to be relatively low (Fig. 11.; Aoki et

Fig. 11. Relative sero-conversion rates of *P. falciparum* SERA3 to SERA6 and merozoite

Since the SERA gene family does not show antigenic variation to evade host immune response (Fig. 7.), there may possibly be another mechanism of host parasite

The host range or host specificity of *Plasmodium* is believed to be restricted, although, primate malaria parasites generally infect multiple hosts. For example, it has been reported that, *P. knowlesi* and *P. cynomolgi* have the ability to infect a wide variety of macaques and human (Coatney et al., 1971); additionally, two human parasites *P. malariae* and *P. ovale* have been detected in chimpanzees (Hayakawa et al., 2009; Duval et al., 2009). It may be probable that duplications of Group IV SERA genes that occurred frequently in both primate parasite lineages may be associated with host range expansions. To date, no experimental support

As described above, the molecular function of SERA genes in each group, the relationship of immune evasion mechanism and the SERA gene family, and the association of host range with Group IV SERA genes remain important issues that needs to be addressed. The importance of SERA genes in parasite egress and their role in host-parasite interactions

This work was supported by **KAKENHI (18073013 and 20390120)** from the Japanese

serve to propel further studies in understanding this multigene family.

Ministry of Education, Science, Sports, Culture and Technology.

surface protein 1 (MSP1) in a malaria endemic area of Uganda.

evasion/molecular mimicry/interference or competition.

lends credence to this speculation.

**7. Acknowledgment** 

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**1. Introduction** 

counteracting hormones.

adult development.

**1.1 JH and JHAs: Insecticidal use of hormone agonists** 

**18** 

**Molecular Evolution of** 

*The Ohio State University United States of America* 

**Juvenile Hormone Signaling** 

Aaron A. Baumann and Thomas G. Wilson

Insect development proceeds through a series of discrete developmental stages called instars. During hexapod evolution, the development of complete metamorphosis introduced a novel mechanism for separating feeding and reproductive stages (Truman & Riddiford, 2002), facilitating the tremendous evolutionary success of holometabolous insects. In contrast to hemimetabolous insects, which progress through a series of instars that appear as smaller iterations of the adult form, holometabolous insects proceed from egg to adult through a progression of isomorphic larval instars and a pupal transitory stage. In each case, the physical boundary for growth during an instar is established by a chitinous exoskeleton, which must be periodically shed. This molting process is under the control of two

Toward the end of an instar, a pulse of the insect molting hormone, 20-hydroxyecdysone (20E) initiates a transcriptional cascade that carries the molt to a subsequent instar. However, it is the interaction of 20E and the sesquiterpenoid juvenile hormone (JH) that governs the developmental outcome of each molt. During larval development, an elevated JH titer and 20E directs the sequential progression through larval development until the final larval instar, when the JH titer substantially declines. The removal of circulating JH facilitates a 20E-directed developmental switch that initiates the metamorphic molt. Thus, it was proposed that JH can modulate 20E activity, maintaining the status quo during pre-

Since the first chemical analysis resolved the sesquiterpenoid structure of endogenous JH (Röller *et al.,* 1967), several homologs have been identified, each bearing opposing, terminal epoxide and methyl ester functions. Variation in the degree and identity of alkyl group substitution at C3, C7, and C11 along the carbon skeleton defines the homologs. The evolutionary importance of multiple JH homologs is unclear. JH 0, I, II, and III have all been isolated from lepidopteran insects, whereas JH III, the presumed evolutionary precursor to the higher homologs, is found in all insects. JH bisepoxide (JHB3) has been identified as a product of the corpus allatum (CA) in higher Diptera including *Drosophila melanogaster* and *Sarcophaga bullata* (Richard *et al.,* 1989; Bylemans *et al.,* 1998)*.* Nearly identical in structure to

JH III, JHB3 is distinguished by an additional epoxide group spanning C6-C7.

